Citroen C5 brake pad mystery?

My 2007 Citroen C5 estate keeps getting the "brake pads worn" alert and beep regularly (in fact it won't stop beeping and is very distracting whilst driving).
My brake pads are almost new (changed less than 1000 miles ago and show little wear) and the sensors have also been checked by my garage and are clean. It has also been hooked up to the computer and there are no diagnostic codes either. Does anyone have any suggestions as to what could be the problem?
Thanks :)