What do some people think DW did? Walk into the local 7/24 hour Wal-mart Super Center and say to himself "Hey, I think I'll buy a Mattress Pad"...oh yeah..I can just picture that!
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If this wasn't a spur of the moment abduction...if he'd been stalking and waiting for the opportunity he indeed COULD HAVE purchased the thin cheap fitted mattress cover days or weeks in advance... that's just speculation..