Per mike:
I have one of those somewhere. Sold by radio shack decades ago.
Some kind of tilt sensor on the mailbox door with a radio link
to an inside receiver. Seemed like a good idea while at the
garage sale. Just one more useless gadget. Never bothered to install it.

My #2 daughter gave us a little radio-based system: transmitter/motion
sensor sits in the mailbox; receiver lives in the house and beeps when
the mailbox is opened (motion sensed).

I thought it was the dumbest thing I had ever seen: how on earth could a
AA battery-powered transmitter possibly get through an enclosed steel
mailbox and transmit to the inside of a house well over 100' away?

Must be totally junk, right ?

Beeeeeeg surprise: it works like a charm and the batteries last for
months on end.
--
Pete Cresswell