I have a bottle cage on the upper side of my downtube, and yesterday I noticed that one of the screws has gone missing. I don't know when this happened, but it could have been months ago. I often leave my bike parked in the rain, so I'm now concerned that water has entered the downtube through the exposed screwhole and settled in the frame, which is made of steel.
How can I check if there is water ingress and minimise any damage? My first thought is to turn the bike upside down for a few hours and let any water drain out, but is there a better way and/or more I could do? I would like to stop any rust in its tracks.