Fuel rail pressure - how long should it hold?

My factory service manual states fuel pressure should be 50 psi with the engine running at idle. This is a six cylinder 2008 Subaru 3.0R engine with a return-less fuel line to the injectors.

Connected up a parts store fuel pressure gauge that shows pressure is on spec with the engine idling. However it drops 10 psi to 40psi after the engine is off for three hours. Overnight pressure drops to 24 psi.

What is acceptable fuel rail pressure drop with the engine off please ? Also wondering where the fuel goes to decrease the pressure as there’s no gas smell unless it’s working it’s way back to the tank or thru the injectors.

EDIT: Thanks for the responses. I carefully clamped the incoming fuel pipe from the pump using vise grips with rubber hoses over the jaws to re-run the test. Fuel pressure now holds much higher for longer so guessing that the check valve or in tank fuel pressure regulator is getting tired.