I have had a similar "problem". I don't really consider it a problem though. If I top up my coolant the level will drop pretty quickly, within a few weeks and the hold steady. It will stay steady for a year at least. There are no leaks, the engine never overheats or causes any trouble. If I top it up it just seems to self regulate somehow.
So now I don't worry about it, I have a bottle of premixed coolant I keep in the boot, just in case it ever gets really low, but otherwise I don't really worry too much. Probably about every 6 months or so I give it a little top up, I know the level will drop by itself but it makes sure that it hasn't lost any more.