What were you shooting? We have pheasants up here, wandering around the garden like chickens. Snipe and red-legged partridge as well within 100 metres of the house - we have them. We love them! Yum!
OK, back to the air-filter cover - seem to have worked out that it's one of the fbh hoses that gets clipped to the side of the cover. So the chances are that you don't have a fbh???
If there's air felt coming out I would put a rubber grommet in the hole or a piece of tape over it. Not for any particular reason other than the hole is superfluous.