I have had similar problems with CCTV in Marple. I once came home to find a very well known local youth (I say youth but its pushing it a bit, he must be well into his 20s by now yet still acts the same as the next generation on scallys! ) in my house. I gave chase to him (maybe stupid but it was broad daylight, I knew him as my brother went to school with him, and the anger seemed to take over). I chased him all down the precint (from the Pineapple right through to the other side near Natwest), then across the road, through the co-op carpark until he started running through the park. I gave up at this point as there weren't many people about and he isn't the nicest of characters.
Anyway, to cut a long story short, non of the CCTV cameras we ran past showed him or me...what are they looking at then if it isn't people?!?!