If there are no visible leaks, then check your exhaust for white smoke. The coolant is going somewhere, and I'm concerned it's being burned off due to a compromised head gasket.
I suppose another possibility is it may be mixing with engine oil. I'd check those three things since coolant/antifreeze doesn't just magically disappear - especially since it's supposed to be a sealed system.
-Check for smoke from exhaust (when your fiesta is at operating temp, you should not normally see any smoke).
-Check the engine oil make sure it doesn't look milky.
-Check for leaks, but I'm sure you have already done so.
-Also I should mention that water pumps have a "weep hole". The coolant loss from the weep hole is very minimal over a long period of time