It is a two sink cabinet with the second sink in bathroom having a very limited smell. Nothing coming from the plumbing drains. Nothing from the bottom wax ring of the toilet. Nothing coming from the vent. HEAVY in the morning, smells like a vinagrone (insect in southern new mexico). Strongest odor comes from under the far left sink under cabinet. Have poored water through top ceiling vent with no issues. I've event cut a 6 x 6 inch hole in back of cabinet to see if an insect or animal was dead, nothing.

Need your help folks. What else could it be?

Smells are the hardest to locate when you're not there. Have you tried taking a rag soaked in bleach and washing down the insides of your cabinet? Let me know if that helps. Good luck, Tom