Anyone here have to top off coolant once a year?...

[Boxtaboy]

I've noticed that my Boxster's coolant expansion tank level drops from the full mark to halfway between full and min level about once every 10 months, so a very slow lowering of coolant level over a long period of time. Temps are always in normal range and I don't see any condensation in tank area or loss of coolant below pass tire. Can there be a leak that slow, or does some coolant exit bleeder as a normal process? Engine oil is not showing any indication of coolant getting intermixed, so don't think there an internal leak.