This vent, on both sides, is "7" shaped, if that makes any sense, where the long leg of the 7 is roughly vertical (leading up from the master box space) and the short leg (to the exit vent) points slightly down and towards the rear of the aircraft. Thus, when in forward flight, air from the master box area is indeed extracted. If you're washing the aircraft, squirting a hose (or even jet-wash) at that area from anything aft of the vent is plain foolish.
Having flown many hours in the high hover in pi**ing rain, it's not a problem, as you'd need some very interesting airflows to get any water into the vent.
ECD put out some advice (3-4 years ago?) to cover the vent when washing the aircraft. Actually, so long as you ensure any water that goes near the vent is only directed from the front, or above, and along/down the aircraft skin - (i.e. as rain would go when on the ground, or in flight) you shouldn't have any problems, vent covered or not. That's from approx weekly washing experience of 5 years + on an aircraft flying 1000+ hrs per year.
The other area to watch out for is the air ducts under the chin - squirting water in there may affect the air blower, as it's mounted with the electrical bits
under the motor. The result is that any water sitting static in the motor fan area may seep its way into the electrics & the fan eventually stops playing. Fit the suppplied blanks while washing & you'll be OK.
