fwiw; I've seen the mirror trick done fixing a wobbling wall cabinet in
a bathroom. Took it out to see where I was going to drive nails or screw
into tooo...
Backside of the mirror looked to have two coatings if I recall correctly,
silver with black on the far back. The black was removed/scraped off messy
but left a neat non glossy silver spot.
From the from reflective side of the mirror it looked like a water spot
discoloration on the back side. Maybe about the size of the tip of a new
pencil eraser.

guy lived alone, must have spied on his guests because it looked into his
own main bathroom,
he would have had to yank that cabinet out and stick his head in-between
wall studs to see anything if he used it.

"m Ransley" wrote in message
...
For a mirror to be transparent a light shown from behind will show this
if it exists, which I dought. Removing the silver will be evident you
would see it. Id say you have a cheap bldg and crappy tennants that cant
see you but hear you and harass you. A camera would need a hole apx
1/16" look for the hole or forget it you truely sound over paranoid.
Why would they go to the expense? they would not as I see it.