Jeff I would think that this would be very hard to detect, unless your looking at very original preserved type cars. I'm thinking the glue thats left would be from areas that didn't come into contact with the trunk mat. Any thing with age would look like rust or were a trunk mat may have sat over the years and has bonded its self the the floor. like I pointed out, seen on '67 shelbys vs regular production mustangs. Not too much on the top of the trunk pans, just down by the gas tank lip area. The similarity of the glue pattern, not overly neat that you would assume would be done by an owner? just find this strange?