[Forestville]

Thank you to everyone that replied to this. I double-checked and there definitely was not a vent, check valve or anything inline going from the sink tail to the other sink and then to in to the basement. Because the drain pipe went all the way up the sink drain pipe with no room to raise it up, I decided it was worth a try to take out a 4" chunk, and put in a rubber coupler. This raised the crossover pipe a good 3/4" and that did the trick. Haven't had a hint of smell since. So my theory that the gray water was sitting in that 4' of crossover pipe was probably correct.

Thanks again for your responses!