There are written instructions to the elders to first call the police ic they find vandalism or damage at a Kingdon Hall. If they contact the Watchtower Legal Department first they will be told to report it to the police.
A broken window can be caused by vandalism or a bird hitting it or even a stone flicked up from gardening equipment. Regardless, call the police.
In vandalism situations it is acceptable for elders to get the help of their wife or a sister to document the damage and help make a report. Why? Because it is a crime and a police matter. The 'two witnesses'becone the sister and brother who discovered the broken window, not who witnessed the window being vandalised.
If a local xjw or apostate is known to have vandalised the hall in the past he or she can also reported to the police despite no 'witness.'