I used to teach here, a former elementary school and, later, alternative high school. Building is not currently in use, but some area bus drivers leave their cars here while on their routes and park buses here at night and on weekends. Muggle activity, therefore, would likely occur just before and after morning and afternoon school bus routes. Weekends might be your best bet.
You are looking for a small plastic container with log. Two presidential gold dollar coins for first-to-find.