Gave bike a good spring cleaning (not washed) and chain on bike lube today. Much to my surprise when I turned it upside down in the stand some water dripped out! Not sure where it might have got in. It is a threadless headset and has not seen any water for almost three months. Anything I can do to keep it dry in the future?

The bike, turned upside down had water run out the headset. So, if there was any water (tubeset condensation for example)collecting in the lowest point on the bike frame, the BB, it traversed up the tube, into the head tube, and out the races. Sounds like it could happen and be nothing to be concerned with. In the future, turn the bike upside a couple times a year to remove any collected water.

Or better yet drill a weep hole at the lowest point of the BB shell to keep water from collecting there. A little nail polish or Rust-Oleum-type of paint will keep the raw metal from rusting.

There is no seal that mother water has yet to be stopped by. Slowed down for sure but completely blocked, no. So periodic removal of water is a good preventative maintenance. The easiest is to pull your seat post and tip the bike upside down. Pulling the fork and BB are also great ideas as water doesn't always leave the bike's interior where you give it the chance to. Andy.

Of Course Most fork steerers are Open to the wheel end.

But most steerers are not open to the headset and/or the frame. No, the water that came out of the headset likely came in between the post and seat tube then migrated up the down tube to the head tube. Since most frames have either completely open passages or vent holes in the back side of the head tube the water get between the head tube and the steerer and comes out through the headset bearings. Andy.

More 20 guesses ... OP? Ok I'm not there... IN the headset bearings? come in the space between head & steerer tubes?