Another Polo central locking issue!

Hi, I came across this forum whilst searching for an answer to my problem... you all seem very knowledgeable so I hope you can help! For the last few weeks, when I have gone to get in my 2001 Polo I have found it to be unlocked. Now to start with I was horrified to think that I had forgotten to lock it (central locking activated by a button on a separate fob to the key) but on Saturday I made sure I checked the door handles before going to bed... sure enough on Sunday morning it was unlocked again! So now I know it isn't my doing, but have no idea why it would be unlocking itself. I have never had any problems with the central locking before, but noticed recently that the 2 click window system keeps deactivating (which suggests the battery has disconnected according to my manual?). It was serviced about 4 weeks ago and they found no problems but reset the windows for me.

Does anyone have any ideas about this? I am a clueless girl who just wants her car to stop misbehaving!! Thanks :)