My first stop would be a pressure test, as suggested by jrk above. With a water loss of this magnitude I would expect that to quickly give a result. If the pressure test is done but no leak is found, DO NOT immediately attempt to start the engine. In a scenario like this where coolant is being lost without any visible sign, it's likely the head gasket is blown and, after pressure testing, wou may well have a cylinder with a fair bit of coolant in it. If you crank the engine immediately it'll hydraulic lock and could do all manner of very expensive mischief! Pull the plugs first and that way, if there's coolant in a cylinder, it'll be "squirted" out (which will also help by indicating which cylinder the blow is affecting.
When the garage checked it over did they do a coolant "sniff" test? This involves using a fluid in a tester tube to check for products of combustion in the coolant. Of course the coolant should not have any products of combustion in it so if any are present then you've almost certainly got a blown head gasket. The kit is pretty cheap - in the grand plan of things - to buy, so many garage workshops will have one and it's a quick test to do and doesn't involve dismantling components, so shouldn't cost much . Here's a rather good video which describes how it works:
Most of the common problems have been mentioned in the posts - the little plastic pipe on the thermostat being a favourite - above but it's worth knowing that the metal coolant pipe which runs across the front of the 1.2 engine (is your's the four cylinder 1.2?) and is obscured by the exhaust manifold so difficult to check, can suffer from localized corrosion and can leak.
I would reinforce what's been said above about avoiding letting the coolant level drop - jrk's idea of monitoring heater output and stopping immediately if it starts to blow cold is a good idea if you really must keep driving it - however, with this amount of coolant loss I fear you are likely to be caught out and it would be better to properly sort the problem. These are robust engines (I'm talking the 1.2 FIRE engine - I have very little experience of the Twin Air - but really don't like low coolant levels (which will quickly result in head gasket problems) I also agree with.
I find myself agreeing with PB above about head skimming. I am lucky enough to have an engineers straight edge and I use it to check the flatness of both the head and block surfaces if I have a head gasket to do. Generally I find blocks don't need any more than a good and careful clean up to remove old gasket material etc - can't actually remember when I last saw a block needing to be surfaced - but heads always need to be checked carefully. However I've done a couple of heads on the Pandas over the years and none needed facing. Proof of the "pudding" being they ran perfectly after reassembly - one went off on a trip to Salisbury only a week after being done! I think heads are often needlessly skimmed in "the trade" as a safety precaution as the repairer can then say, "Not my fault gov, we had the head skimmed so it's not negligence that it's blown again"! Also gives the boss peace of mind where he may slightly doubt the ability of a member of staff to effectively find fault with the head? The only exception to this I'd make is where there's any doubt that the head may be porous or cracked then it should be pressure crack tested by an engineering workshop and they may insist on it being faced so they can check it.