The 'machines' may have been maintained, it doesn't mean the filters installed on the waterline before the machines were. Properly functioning UV filters kill

The 'machines' may have been maintained, it doesn't mean the filters installed on the waterline before the machines were. Properly functioning UV filters kill bacteria, there would be no way to introduce bacteria into the system from the waterline if the UV filters were working adequately (assuming the upstream carbon filter wasn't acting like a reservoir exceeding the specs of the UV filter also due to lack of maintenance). Since the other machines elsewhere didn't have these filters, it's safe to assume nobody had them on the maintenance list and serviced them.