Your heater valve could also be crapping out. I was having coolant loss but couldn't find anything on the ground, and oil analysis should nothing leaking that way. Eventually I did see some coolant on some hoses in the engine compartment and traced it back to the heater valve, right by the firewall. Apparently the leak was large enough to result in cooland loss over time, but small enough not to hit the ground, and only be dissipated while driving. I can't recall what I paid to replace the heater valve, maybe $250 parts and labor, and while supposedly it's a DIY job, I found getting to the valve was a real p.i.t.a. I think my mechanic had to get at it from underneath the car. Anyway, that was the problem in my case, and as with other cooling system-related problems with these cars (water pump, radiator) it's not all that uncommon.