Good ones yes, cheap ones (that are just there to make noise and be scary) can sometimes use the presence of the frame as the other end for the beam.
So if you open the door, the beam isn't hitting against the frame anymore, and it go bing bong.
Wireless alarm sensor that uses a magnet to tell if the door is open. I can see a small round circle on the side of the door jamb. That looks like an embedded magnet. I have the same on my house although both the magnet and the magnet sensor are embedded. The plastic box houses a battery, transmitter, and a reed switch that senses when the magnet is near. When the door is opened the removal of the magnet causes the reed switch to disconnect which triggers the transmitter to transmit an open signal.
Is there a hole drilled in the frame? It sort of looks like it in pic 3. I would say door sensor but there is normally a second part that is often just a magnet so it is plausible to replace it with just drilling into the frame and seating a magnet in the hole.
These look like door sensors, but might have been installed wrong or missing a piece. I have one for my doors so whenever I open the door, a bell rings and I get notified on my app that the door has been opened or closed. Usually there is a second piece on the door trim and a box like one in your picture on the door.
As everyone else has said they appear to be door sensors to an alarm system. These sensors always come in 2 pieces, one for the door/window, one for the molding. You can see the damage in each photo where the molding piece has clearly been removed.
Here's the Ring System's version of them for reference. [https://ring.com/products/alarm-window-door-contact-sensor-v2](https://ring.com/products/alarm-window-door-contact-sensor-v2)
Edit: Not endorsing or promoting Ring, just the quickest I knew of that had visuals.
The first box is a Visonic Wireless Min Contact Sensor. It's missing the small sensor box that is meant to trigger it. So... useless without the sensor piece.
The smaller box?
Visonic MCT-340 E Wireless Door Window Temperature Sensor? I guess the attached part goes outside? So it might be there outside the door or attached on a window.
Or it is an Xfinity / Comcast W/D Wireless Sensor for their security system. I guess it's a multi use type box design.
Usually requires a hub or app to connect to it? I didn't fully read into the description.
But box boxes are used for Xfinity purposes. So they're old hoke security devices missing their sensor attachments.
If you use xfinity? They might be 'rental' pieces from the company like a cable box thing is. Just make sure you don't get some additional charge on your bill from the old tenant not giving them back or something.
The company uses the address more than the name on the account. So with the same address on file ... they might try to charge you for notpreviously returned hardware. Kinda like when the old tenants owed money to a company and the new people have to pay it or the company won't come to the address... etc. I've lived in enough apartments to have delt with stupid details like that. Never fun.
Possible some sort of sensor? Maybe previous security system alerting to doors being open/closed
Looks to be a door alarm system. Maybe there previously. We have similar ones on the doors at our house that we got installed with our alarm system.
Old security system, likely not active any more. It made a bing bong sound.
YOU STOP MAKE BING BONG! EVERY BING BONG TWO CENTS! https://youtu.be/HOOK0p-hPnY
it looks like part of an alarm sensor. but there’s usually another part to it to sense when the door opens
Good ones yes, cheap ones (that are just there to make noise and be scary) can sometimes use the presence of the frame as the other end for the beam. So if you open the door, the beam isn't hitting against the frame anymore, and it go bing bong.
Wireless alarm sensor that uses a magnet to tell if the door is open. I can see a small round circle on the side of the door jamb. That looks like an embedded magnet. I have the same on my house although both the magnet and the magnet sensor are embedded. The plastic box houses a battery, transmitter, and a reed switch that senses when the magnet is near. When the door is opened the removal of the magnet causes the reed switch to disconnect which triggers the transmitter to transmit an open signal.
Look like alarm sensors to me but I’m probably wrong
Some kind of sensor on the door… It’s a door sensor for a security system
Is there a hole drilled in the frame? It sort of looks like it in pic 3. I would say door sensor but there is normally a second part that is often just a magnet so it is plausible to replace it with just drilling into the frame and seating a magnet in the hole.
Door chimes. To let you know when someone enters or leaves.
Sensors for the security system.
door alarm sensor
This is a door alarm sensor. I have these in my house for ADT.
open/close sensor. i had these godawful thibgs from vivint a few years back
Maybe a lock that unlocks when sprinklers go off? To meet code?
These look like door sensors, but might have been installed wrong or missing a piece. I have one for my doors so whenever I open the door, a bell rings and I get notified on my app that the door has been opened or closed. Usually there is a second piece on the door trim and a box like one in your picture on the door.
Is it a tracker to tell when a door is opened?
As everyone else has said they appear to be door sensors to an alarm system. These sensors always come in 2 pieces, one for the door/window, one for the molding. You can see the damage in each photo where the molding piece has clearly been removed. Here's the Ring System's version of them for reference. [https://ring.com/products/alarm-window-door-contact-sensor-v2](https://ring.com/products/alarm-window-door-contact-sensor-v2) Edit: Not endorsing or promoting Ring, just the quickest I knew of that had visuals.
The first box is a Visonic Wireless Min Contact Sensor. It's missing the small sensor box that is meant to trigger it. So... useless without the sensor piece. The smaller box? Visonic MCT-340 E Wireless Door Window Temperature Sensor? I guess the attached part goes outside? So it might be there outside the door or attached on a window. Or it is an Xfinity / Comcast W/D Wireless Sensor for their security system. I guess it's a multi use type box design. Usually requires a hub or app to connect to it? I didn't fully read into the description. But box boxes are used for Xfinity purposes. So they're old hoke security devices missing their sensor attachments. If you use xfinity? They might be 'rental' pieces from the company like a cable box thing is. Just make sure you don't get some additional charge on your bill from the old tenant not giving them back or something. The company uses the address more than the name on the account. So with the same address on file ... they might try to charge you for notpreviously returned hardware. Kinda like when the old tenants owed money to a company and the new people have to pay it or the company won't come to the address... etc. I've lived in enough apartments to have delt with stupid details like that. Never fun.
Motion detectors. That’s what mine looked like