On Tuesday, September 5, 2017 at 7:13:19 PM UTC-5, wrote:
I wish you found the culprit. I'm going through this now. Newly renovated bathroom and I can't even use it because the odor is so bad. for me, I have a double sink vanity. The smell is 100% coming from the left side cabinet/sink but after deep cleaning, the smell just got stronger. It's as if the smell gets worse whenever I run water in that sink. And yes, it only smells in there, it doesn't travel to the hall.

Since it smells worse when you run water, perhaps there is mold in the drain pipe. Pour mildewcide or bleach in the drain, and in the overflow hole. Another possibility is perhaps the vanity was made with bad particle board, in which case a coat of KILZ should seal in the odor.