Arrived on site and checked in with customer. Was told that water was leaking from corners of supply plenum. Proceeded to roof and began inspection. Found that drain was working properly and pan was not overflowing. However, discovered that supply side panel had a screw partially undone and a gap that may have allowed water in. Confirmed with customer that leak only occurred during a time of heavy rain. Re fastened screw and verified seal at supply panel. Collected payment for services and checked out with customer. Further services not desired at this time. System was operating, draining, and sealed at the end of the service call.