If you have any bushes/undergrowth around then have a quick nose in there - it might just be some pathetic little specimen who finds it amusing to tamper with people's possessions. Had a few problems back in 6th form with moronic rat-faced 9th year who thought it was funny to steal valve caps (and doubtless fling them into bushes).
It just strikes me as a rather odd thing to pinch. I know scrap prices are high but would they really think it worth the risk of a beating (more likely than a decent judicial punishment methinks!) to get a few quid from a dodgy scrappy?