Depending on where you're at and the water/air temps you could be getting condensation on the inside of the hull.
Also the cockpit lockers drain into the bilge. I know if I get a hard driving rain I'll sometimes get some water in the port locker but I'll be damned if I can figure out how it gets in.
95 Catalina 30 Island Time
“Only two sailors, in my experience, never ran aground. One never left port and the other was an atrocious liar" - Don Bamford