Smell coming from guest bedroom

House was built in 2000; new water heater installed 3 days ago. Furnace and AC are original to the home. Moved in 2 months ago.
Since we moved in, we have noticed a smell coming from the guest bedroom. It is only present in the guest bedroom. The bedroom is on the main level, adjacent to the garage, but is on slab. The basement is across the hall and below. The smell has been described as exhaust (no, cars are not running in the garage when the smell is present), plastic burning, and jet fuel. It seems stronger since the new water heater was installed and the plumbing to the garage sink was turned off. There are two plumbing pipes in the closet in that bedroom, but no apparent leaks (we have not opened the wall to physically look at one of the pipes, but the other is exposed. The smell dissipates after some time, but comes back almost daily. No pattern of water usage, etc have been noticed. P-trap in garage sink is not the issue, as we have run water consistently. CenterPoint Energy has tested for gas leaks and found nothing.