Hello, All and @davsav
I would think external AAA batteries should power a sensor for a very long time as long as they are hooked up to provide the correct voltage.
Below is a link to an externally mounted mailbox contact sensor enclosure I meant to post a while back but forgot to. The discussions in this post reminded me.
Some of you may find it interesting.
Externally mounted contact sensor housing for metal mailbox
Victor Maletic