The best advice I can give is to call the mfg's customer support. Don't guess, since if it's not a drain hole, and you make one, that's bad. Just call.
As an aside, on the JRC's I've had (one was labled Raymarine,) there is a relief valve for air pressure. When I lean on the dome, it bends and I can hear air going out. Since the dome rebounds, I assume there is air coming back in. No obvious drain hole. Remember there's electronics in there.
boris