The most likely thing to go wrong is always the most likely thing to go wrong.
First top the beast up on Sunday morning, when it's been sitting cold all night. That's your true level.
If it drops over a week or two (depending on how much it's used) then it's either getting burnt (Head gasket) or it's leaking somewhere.
Leaks is the most likely... so check the hoses. If you jack the front of the car up you'll be able to get your moosh in there next to the radiatore... Any leaks will be more visible from below. Look for streaks of "white" or "pink" crustiness of dried coolant coming from the ends of the hoses and running down hill. If you have Jubilee clips, give them a 1/4 turn of tightning.
If it's been dry, so much the better, since any wetness you see in there may be coolant.
Check the plate the radiator sits on. If it's minging, wet and rusty that may be because the radiatore is leaking. Check the rad for wetness and also the streak marks of dried coolant. If your rad' looks remotely manky where the core meets the pillars (and you don't need the car that week) give it a good rattle and see if it collapses and coolant runs out of the joints... (new rad' time)
If you can't see any leaks in the engine bay.... have a peek indoors. Feel the carpet under the heater matrix. Do they have a green or more obvious pink tinge...? Are they damp? Could be a leaky heater/connector/something.
Now you can change the cap on the expansion tank in case it's not doing a good job... but you can normally tell since you get a coolant stain on the bonnet liner just above it.. Still.. it's a few quid and may be worth a punt if it looks remotely moist.
If you have no leaks, stains, wet patches or anything... then it can only be going into the motore. It's probably not a major leak so potentially not much to worry about (my sister's Renoort Megane has been knocking back the pink stuff for years) but next time you have a cam-belt change it may be worth just going a bit further and sticking a new HG in there at the same time.
Just top the level up at a frequency that keeps it around half-way. Sometimes having less coolant in there makes it less likely to disappear... but don't run it down to Minimum too often...