Clutch reservoir slow leak

Over the last few months I’ve noticed a small but steady leak coming from the clutch reservoir/master cylinder but can’t identify from exactly where! It would seem to be from the vent hole at the bottom but can’t be sure, it doesn’t drip in the tank but it’s defo weeping! Any had any experience of this?

the levels are all ok ( at least they are now, Triumph brimmed both brake and clutch after a bar change).