Kurt Gavin wrote:
I'm talking about those standard roadside USPS boxes.

Mine's in front of the house and I'm wondering if there's a gadget that can
tell you by sight, or maybe electric trip switch, if there's something in
the box.

Thanks

A good tinkerer can make a simple device like that. Open the door and
it trips a flag. Of course you'd have to reset it every time you got
your mail.