Does your bottom bracket have a vent hole on the bottom?
No mater how tight the clearances, or how well built your bike is, water will find a way inside the frame, mostly through the top of the seat tube. It'll work it's way down to the bottom bracket shell, and if there's no hole, it'll sit there. For steel frames this is an obvious problem as it will cause rust. But even for aluminum or carbon frames it's still a problem, because it'll ruin your BB bearings faster.
My roadie has been sittng unused for several weeks. It's been in the garage. Last night I pulled the BB and replaced it (it hardly turned....), and there was water sitting in the frame. I had always assumed there was a vent hole, but never actually checked. So I grabbed my drill and put one in.