The plug is just below the CompTip joint. Water can pool on top of it. If the plug has failed or has a small leak, the mast can "pump" water into the inner area as the mast cools and heats. You may try to tip out the water, but the seal may not allow water out easily. You can jam a screwdriver into the plug or past it down the side to open up a drain, then re-seal well.