A lot is possible. Could be gear actuator, could be a door actuator, could be the dump valve, etc.
I rebuilt ALL of my actuators and still had an internal leak. Turned out it was the dump valve. By the way, the valve in my airplane was a Parker valve that specs list a maximum psi of 600. Kinda makes you wonder..
As you can see in this pic, the seal was displaced. It sucks it was the last thing I checked, and probably the cheapest fix.
You can read about it here:
Tom McNerney