I appreciate the responses from yourself and Jim Yanik.
Nice sanity checks

You seem very experienced, so let me explain my situation if
you don't mind and get your thoughts. I'm working with limited
available time to prepare this property to sell ASAP, so I'd
rather not deal with this at all. My understanding is that part
of disclosure to buyers is that you must state any problems like
"nuisance neighbors" or fix the problem before disclosure.
Given the responses here, I suppose that I might claim
that I'm not paranoid and that there are no peepholes, so no
disclosure is required. The other approach is to start with
a reasonable amount of investigation and follow where it leads.
Do you think that I should just forget about this?

In response to your post:

I'll try your mirror inspection technique when I return home.
It's a quick check before pulling the mirror, so why not?
I thought that I'd also try turning off the light and angling
a flashlight beam to spot irregularities. Another quick check.
I'm using my time on the road to gather info after suspecting
this problem last week.

I also like the idea of tilting the mirror since it is mounted with
brackets, which I'm pretty sure that I can do myself.

"Tell us more. Is this a standard apartment building, or a converted
single-family house, and when was the conversion done?"

Given everyone's wondering about why I can't just see a hole, I
think that I might skip this problem and focus on the mirror, but
thanks for offering your help.

I might try your idea about sitting in the dark. I'll just choose an
appropriate moment like making some noise rather than waiting
forever.

There are no windows in the bathrooms, but that doesn't mean
that I'm not wrong about the peepholes. I'm wrong about
many things.

Thanks for the very complete, useful post

Dan Hartung wrote:

xyz wrote:
I'm fairly certain that there are peepholes in my two bathrooms
in the walls shared with a neighbor's bathrooms.

I take you seriously; you didn't cut and run on this post, for one
thing. For another, I've been aroudn the internet and I've *seen* what
is out there. Also, there are occasional criminal cases.

I will say that one of our tenants believed that the former landlord,
who lived upstairs of her, was spying, as he always seemed to be right
above her. I've been through that place and haven't found anything. So
paranoia does occur too.

One peephole is behind a mirror. I've heard about this problem in motel
rooms but don't know exactly what's involved. Is it just a hole in the
wall behind the mirror, and the silver on the back of the mirror is
scratched off, or is there something else involved? Is there a way to
locate the peephole without removing the mirror? I want to subtly find
the hole and try to document if it's used.

Usually the mirror's silver will be scratched in an inconspicuous place
like a corner. The other option is one-way glass (aka, incongruously,
"two way mirror"), which would have a different appearance from a
typical bathroom mirror. They tend to be metallic and cloudy. If there's
any light on the other side, you can actually see back through, and you
can also see through dimly if you get up close to the glass.

(I used to work in a place with psych interview rooms.)

If there's really a hole in the *wall*, though, you'll have to remove
the mirror to prove it's there. Mirrors aren't that hard, they're just
fragile, awkward, and heavy. ;-) Find out how it's supported; sometimes
they are just resting on brackets and clipped in place at the top.
Removing those clips would allow you to tilt the mirror out without
removing it completely.

The other hole is somewhere in a wall that's made of something the
thickness of heavy cardboard on my side but finished to look like
drywall. Any ideas on what this material is and how to find peepholes
in this material?

Odd. Like a false wall, you're thinking?

(Tell us more. Is this a standard apartment building, or a converted
single-family house, and when was the conversion done?)

It could be a plywood or fiberboard of some type. What kind of sound
does it make when you plonk it with a finger? Does it have any "give"?
But I doubt it's in the material so much as in a convenient corner or
molding.

I tried turning off the light, hoping that the light
was on on the other side so that I could see the hole, but I think that
they block the hole when not in use and turn off their light and close
their door when using the peephole.

That would be wise, of course. You could "stake out" the room by sitting
there in the dark before they come in, if you can hear them. That might
give you a flash of light as they open the door, telling you where the
hole is.

My thinking is to collect some evidence, then get the police to inspect
the neighbor's place. Any comments that don't involve violence?

Let the police take care of it. If there is any evidence on *your* side
that you can show them, then they will have probable cause for a search.
If not, they will need the permission of the other tenant, or the
landlord. Proving it was used, without evidence like video, would be the
hard part, and possibly not worth it to you. (It will take months to
come to trial, the prosecutor will put you off, etc.) Consider just
moving instead. If you have evidence, the landlord will probably quickly
evict the other tenants, though, so you could wait that out. The
landlord has a duty of care to give you privacy.

Anyway, a sanity check. Are you sure there's no way they could see you
(eating pizza, say) without "peeping" -- e.g. a reflection on the next
door building's windows? That happened to me once -- neighborhood
teenagers would hang out on their deck and even though I couldn't see
them directly, they could see activity in the bathroom reflected in
their windows, generating raucous laughter. We solved it by getting
better shades.