We are having a problem with a musty/mildew smell in one of our
bedrooms and I need some advice. The specs:

Home built in 1987. Had original carpets when we bought it a year
ago, no trouble with any mildew smells or the pads. When we pulled up
the carpets in one bedroom to paint, I saw a hole in the slab that was
mostly behind the wall; the hole is about a foot square and is mostly
under the adjoining room, which is the bathroom. The hole is under
the tub and has a 1" PVC pipe coming out of it. In the bedroom about
6" X 2" of the hole is exposed, just past the carpet tack strip. I've
never had a slab house before, I figured it was an access point for
plumbing. The hole wasn't bone dry but not wet by any stretch of the
imagination. By reaching my hand up inside the hole and looking with
a mirror, I couldn't seen any black or anything. The slab, wood studs
and plate were dry. We painted and had a new carpet and pad put down.
No smells at all. BTW, the bath on the other side is in good shape
with no mildew, rot, etc.

A few weeks ago, a year later, my mother stayed over. She has a
sensitivity to mildew, and, without knowing about the hole, pointed
out that she could pick up a musty smell in the exact location of the
hole. We had two plumbers over to do some unrelated work; one said to
patch the hole with concrete and the other said to fill the hole with
lime, sand and plastic since it was a pipe for methane gas and might
need access. We trusted the later and pulled the carpet back for a
few days to air out the hole while running a fan and air purifier,
then filled with lime, dry sand and plastic, plus sealed down the
plastic with tape. The pulled back pad and carpet and slab were dry
like they were just installed. Then the carpet was replaced and
tacked. My mother couldn't smell anything so I figured it passed the
acid test.

Usually we keep the door closed. I did keep the air purifier going,
though. It's pretty heavy duty with a charcoal filter and atomic
strength fan. A few days later we opened the door and we can really
make out a mildew smell. In fact, we get a splitting headache by
standing in the room. Once the door is open the smell dissipates
quite a bit but remains. My wife thinks the smell is more like wet
sand or chalk-I think it's like a laundry room. I can't figure out
where the smell is coming from since the hole is sealed up. Any
ideas?