FINGERPRINT ON GATE CONTROL BUTTON

IN ALL THE INFORMATION I HAVE READ ON THE TATE MURDERS, IT SEEMS EVERYONE BLAMES ONE OF THE FIRST RESPONDING POLICE OFFICERS (DEROSA), FOR RUINING THE BLOODY FINGERPRINT ON THE GATE CONTROL BUTTON INSIDE THE TATE PROPERTY. IT SEEMS TO ME THAT THE HOUSEKEEPER (CHAPMAN) HAD TO PRESS THE BUTTON TO GET HELP. IT IS MY UNDERSTANDING THE GATE CLOSED BY ITSELF AFTER SOME TIME. IN THE BOOK "HELTER SKELTER", THEY EVEN POINT OUT THAT WHILE FLEEING THE PROPERTY SHE CHANGED DIRECTIONS TO REACH THE CONTROL BUTTON, THUS DISCOVERING THE BODY OF PARENT IN HIS RAMBLER. IM I WRONG? THANKS!