A less nefarious possibility is they cut the seat back for amps and subwoofers to get more sound in the cabin. I'm not a stereo guy, so I can't say what the impact would be.

If you're really nervous you might want to contact your local PD and ask them to run a drug dog over the car. In the 80's they offered to do this for Porsches and M-Bs in SoCal so you wouldn't get popped inadvertantly.