Seems like random water leaks are the achilles heel of these cars. I would defintely have the pull the headliner and replace it. I think you're correct that a drain is plugged. Most of the leaks I'm familiar with are in the front floor boards. I noticed my sunroof drips small amounts of water in rain and when washing, but there is a catch basin for that reason. Seems like they designed them to leak with the expectation to drain the water out. Weird.