How about, as was suggested for the crash in Egypt, something stuffed in a life jacket pouch under a seat?
I was thinking the same. How often are these areas thoroughly checked (in terms of the life jacket being there I mean)? And when they are checked, I would doubt that it would be more than just a perfunctory check (quick "feel" check).
I'm also curious as to how long the aircraft was on the ground prior to departure? Had it night stopped?