Nice to know that the leak has stopped for now.
Only issue i can see with using a sealer in the water system is that it will clog the water ways in the radiator eventually this then leads to overheating and possible engine damage if it is not sorted.
I would look REALLY hard for any rusty looking deposits on the chasis,if you find any then you are near to finding the leak, also check the carpets in the car near to the heater matrix to see if it's damp, if so then the heater box is the culprit.
1 litre Zetec Turbo petrol 74Kw / 100 Ps. as in my avatar.
If coolant is being consumed and neither you or the garage can find where it is coming from, that suggests no puddle on the floor or rusty water marks under the bonnet. That leads me to the conclusion that the most likely source is a leaking cylinder head gasket.
B Max Titanium X 1.0 125 bhp in Magnetic with built in SatNav and rear sensors.