is it possible someones stuck a sticker under ur battery cover of your phone? (the alarm ones) i know i work in ASDA, and i'll often find someones fleece, then decide to put a soft tag in the clothes tag on it, so they can't find it, hilarious when theys a security check on the mornings :)
Just make sure noone has put one of those little sensor things from a product. in your wallet.
I used to do this to mates at high school for when we were in the school library... I would pull a sensor from the back of one of the books and either stick it on their back or put it in their wallet or back pack. I found this hilarious.
I had the same problem a few months ago!.....I have since found out that the muppets at Morrisons did not deactivate my SD card after I bought it........