Last week, when a friend was servicing my 2012 FL2, i noticed seeing daylight at bottom of airbox. I dont remember seeing the hole when i fitted a K&N panel filter six months ago so i know it must have been in place then atleast.

I bought a rubber non return valve from Muddy Mods for £17 including p&p and was delivered today. I've glued it in since photo was taken to try atleast prevent it falling out again. Allegedly, its common for them to drop out.

Drainage flap for any water that gets into the box

I must add, that when it dropped out it was sucking dust and debris through the open hole and the K&N filter is really really dirty on that particular side. Since the filter had only been fitted six months ago its obvious there is a obvious quirk with air harmonics under the vehicle which encourages air/dust etc into the engine bay. I've always noticed, the plastic engine covering tray is always dusty.

I'll check it today to see if the valve thingy is still in place.

Im paranoid it will drop out so i applied epoxy resin to it so everything is solid. Check yours is still in place unless you have a snorkel fitted.