Anything is possible from a lot of food debris and grease and the tank not rinsed good or at all.
If it is not a vent it may be that.
We wash cloths and use softener and shower with gel and shampoo.
We do not let any grease into thee tack with exception of minute amounts, and of course dish washing soap.
I would fill it by washing clothes and showering etc about 3/4+ full, maybe add some water softener and bleach perhaps. Drive it down a long bump road and go though the fill and flush. If that don't get whatever is in there than a long wand on a power washer up the drain maybe. :)