Can fail at any time if the PO or shop did not pre oil the 2 big green "O" rings on the oil cooler inside the cooler housing-the seals can "roll" or deform. Or severe engine overheating. Just a matter of time.
A pressure test may not show up oil cooler housing seal failure. It will pinpoint drips on the outside of the engine.
I would do a leak down test to see of the HG may be leaking. A leak down test will show bubbles in the expansion tank if the HG is bad.
Compression-leakdown test
GL
John