Should I Be Worried?

My parents left for a short break in Bruges, Belgium a couple of days ago. When I came down after they'd left, I found a note from my Mum. It said "Keep back door locked when you are upstairs."

What is it that they aren't telling me? A little local crime spree or some crazed axe murderer? I've kept the back door well and truely locked ... and will be having a word with Mum when she gets back.