This has happened to me twice recently!
Today was the second occasion - left WHSmiths with 2 bags of stuff and the alarm went off. It's a little embarrassing having your bags searched and items checked off against your receipt, in full view of an openly agog, slack jawed with anticipation, public .
What could be causing this? Could it be my bluetooth mobile phone? The items I bought were small things like printer cartridges - no obvious security tags left on....