Beware of checking levels after driving home then comparing to the level in the morning. (it will be lower).
The coolant level will be higher when its hot than when its cold. (as comfirmed by the fact it hisses when you remove the cap if the coolant is warm/hot and not when its cold).
If you keep topping it up to max when its cold then it may be too high when its hot.
This may mean the pressure in the system is higher when at operating temperature as you cant compress water! however if its only a little over max when hot you should be ok.
This could, in turn mean that coolant is forced out somewhere. There are a few places in the system where there are rubber rings, for example on the pipe coming off the bottom of the radiator. (there is also a drain tap here FYI) I would imagine its more than possible for the coolant to force its way out of places like this if the pressure is high enough as there is a single rubber ring used to seal this.
It would also be difficult to see a leak here unless you get under car and remove the undertray and bumper! in fact, the rate you are loosing coolant will be difficult to trace.
The question id ask is this:
When you say the level is low, how low is it?
Is it actually below the minimum marker of the expansion tank? or are you going by the bleeping flashing light on the dash in the morning only?
This sensor (in the expansion tank) is a known
problem, you may find once you have driven a little and turn the ingition off and then on again it stops. (of course its possible the level is low also). But given you are only putting in a couple of cups to get it to maximum id say it may not actually need topping up in the first place?
PS im on my second leon, neither have needed topping up in about 4-5 years combined motoring and both have suffered from the sensor issue. (false reading of low on cold start up).