RESSAC technician arrived at supermarket in Oxnard to perform maintenance on five Trane package units. Technician replaced all filters and noted that when replacing filters for one of the units, the compressor was low on refrigerant. A request was submitted to the company that managed the store to fix this issue. After wrapping up, the technician called the alarm company that managed the a/c controls monitoring system at that location to verify that everything looked good on the computer system.