I know how you must feel, my last car was keyed all down one side before, no good reason, no excuses, a completely senseless thing to do. €180 it cost me to get it sorted out iirc, and not my fault in the slightest. Last time I parked in that area in the evenings though!
One of the crimes it's almost impossible to catch people for, unless you catch them in the act. I don't understand it though, have these people nothing better to occupy themselves, are their lives that worthless that they have to destroy other people's property?
I guess you guys have asked the centre for any cctv footage they might have?