A nasty swampy odor was the sign I noticed... like what the heck died in here! Had just returned from a 4,500 mile road trip. Looked all over for the lost 3/4-eaten sandwich stuffed under a seat to not find anything. Changed the cabin filter thinking it turned funky. Then wet-shampooed the seats thinking my friend had spilled something. It was the next long trip a year later that I found condensation on the underside of that wonderful Huskey mat in the rear seat area. Once again didn't think much of it, thought was likely sloppy filling the ice chest when leaving the hotel. But no, last thing I would suspect is a leaky rear cab wall.Without having to initially do these steps are there any early indicators? Such as feeling of dampness in the vehicle, smells, condensation on the windows?