[Brian in Tucson]

Not all coolant tank leaks are huge. Have you felt around underneath yours? A small leak would account for your problem and should show up a moisture in the carpet. I've been chasing down leaks on my 01 since I got it last summer. Everything in front of the firewall is sealed up, but I still have one from the tank ( which I replaced last fall.) If need be, I'll replace it again, if I can't seal it up. But it's gonna require pulling the tank off (again.) These cars are SO fiddly!

If you do have to pull the tank off, plan on replacing the two segments of hose in front of the firewall that connect to the metal tubes. The hose to the oil cooler has quick connects, mine leaked so I replaced it with a permanent junction. Much better!