sewage like smell in car when driving any idea?

[Stunin_Stud]

I sometimes get a faint sewage like smell in the car. sometimes I think its coming from the outside environment but Im starting to think maybe its from the car. My car is not driven often could this be behind the reason or would it be some sort of leak? its not too bad and goes away after a while but just curious if anyone has had this and knows what it could be?