Add U/V dye to the oil, run it up for a considerable time, and strip down - U/V never lies....

When you said under the oil filter housing I presumed you meant below, not behind it.
I've replaced several cam cover gaskets, and 1 cam cover for leaks, never an oil housing/cooler though.....

"Caveat time" = It's you're job, only YOU can make those decisions.