Yeah, sounds like a former store display. I'd wonder if some store employee (or "customer") stole it and resold it. I've often wondered who owned those display model Kindles, Amazon or the store. If the store, they may have had the right to resale it, but I'd guess it was against their contract with Amazon.
The store displays usually have some white print on them that say "NOT FOR SALE", usually on the top to the right of the logo. Was there anything scratched off in that area?