Guest bathroom has a foul odor originating underneath the sink. Started about 2 months ago. It doesn't really smell like decaying flesh, but it doesn't smell like sewer gas, either.

Sink is vented with a Studor AAV vent.

Since that bathroom is rarely used I assumed the trap ran dry. Added some water, issue persisted.

None of the pipes show any sign of of leaks. No mold present. No leaked bathroom products.

I replaced the AAV. Issue persisted.

I then assumed it was a dead rat or something underneath the sink.

Went into the crawl space, no smell underneath the house near that sink.

I'm at a loss at this point. It's not the AAV (I verified by rubber-banding a plastic bag over the AAV and the smell persisted), and it can't be the P-trap.

Any ideas?

dead rodent?

If it were coming from the pipes, putting your nose near the sink drain would tell the tale (might need to drain or remove the P-trap first).  At least you would know.

If it's not that, is it in the walls or other spot.  

Do you have any kids that might have questionable aim?

Happened at my buddies cabin. The undersink vent was loose. Just glued it back on and that fixed the problem.