Well..... It appears to be; a backed-up a/c drainage pipe leaking into ply-board and/or insulation under the a/c (in the intake vent), that is causing the problem.
The pipe has been unclogged but now I still need to eliminate the odor. I can't remove the ply-board, or the insulation, easily w/o removing the HVAC system or the drywall. Any suggestions? Should I spray the area in the intake vent with bleach periodically while it's drying out? I sprayed some odor eater stuff in there but it only works for a couple of hours. Do you think it will just dry-out on it's own and stop smelling or will there always be a mold problem unless I rebuild the whole thing? I'm in Central TX and it's been fairly humid lately.
If you're wondering why I thought it was the bathroom, it's right across the hall from the a/c.
Thanks for all suggestions.