I had come across some very cheap (both in price and in quality) door and window alarms. Four for $4 at Menards. I saw them and thought, “oh, a housing and a reed switch.”
These things are quite cheaply made. No proper reed switch at all. Just a very cheap impersonation of one. Granted, it works – after you tear the thing apart and adjust the contacts. The first one out of the package didn’t work at all. I twiddled with it, and got it to work reliably.
I like the leave the windows open here. They can only be open a little bit so the cat
s can’t claw up the screen or force their way out. I figured if I put up two of the magnet half of the sensors, one for the door/window closed, and one for the partially open position, if somebody decided to let themselves into this unlocked door/window, I will hear about it.
I also have cut bars that I drop into the window/door track. I think I will cut those a bit short, so if sombody gives the door/window a try, I will hear about it. They will still be long enough that you can’t fit an arm in far enough to lift them out of the way.