This is not going to be the usual thread. I know I can cool down the board, close the vent, take it outside and then wait for the air to expand to show the leak. However, I'm too impatient.
blowing air through that hose (just using my lungs) I was able to find the pinholes right away. Now, why did I get pinholes? Well, that's for another thread.

Well, I found a couple of micro holes on the deck. There was no sign of damage/soft spots so no clue how they appear (maybe they were there when the board came out of the factory?)

Excellent idea. You see any dried salt in those areas day after sailing? If you have multiple in areas of no damage, its likely all boards have pin holes???

PS: Nearly 30 years ago I saw the Board Lady screw a self threaded piece of plastic tubing into my boards vent and using a vaccum pump determined it was sound.