They’d spent the morning tidying up a forest path, when 12 o’clock came and it was time for lunch. But what to do with the chain saw? They spotted a convenient tree and chained the saw to it before trotting off to enjoy their sandwiches and coffee after a hard morning’s work.
When they returned an hour later, the saw was gone.
Even they could no doubt deduce how the crime had been committed. The thief had simply started the saw and used it to fell the tree it had been so carefully locked to. He helped himself to the thoughtfully provided petrol can as well.