Living in Silicon Valley, it's hard to get mold since it's so
sunny and dry most of the time.

Had guests over (don't ask) who needed to do wash and we
washed what turned out to be their moldy stuff with my
towels.

When they left, we noticed the musty odor in the bathroom, so
we narrowed it down to the towels washed with their stuff.

No problem. We just re-washed (and re-washed) the towels.
Yet, in the bathroom itself, even after almost a week of
airing out the tile bathroom with the windows wide open,
the faint moldy smell still exists, ever so faintly - but
it's definitely there.

Googling for how to locate the source, I find the keyword
MVOC (microbial volatile organic compounds), e.g.,
http://www.emlab.com/s/sampling/env-report-04-2006.html
http://blackmold.awardspace.com/mvocs.html
http://www.ncbi.nlm.nih.gov/pubmed/18333991

End result? It's a complicated mess. But, yet, I still have
that faint yucky musty smell of mold sticking to the membranes
of my nose whenever I enter the tile bathroom.

The bathroom never had this smell before; and it won't
get rid of it now. It is a 100 tile bathroom, very dry,
on the third floor, with tons of sunlight and wide open
with no wet anything anywhere.

What on earth is the mold sticking to?
Any suggestions?

PS: I'm wont to snap a picture but I don't know how to snap
a picture of a mouldy smell...