It’s unlikely to be leaking into the hot water cylinder as you say the system get’s air into it, and that won’t be possible if the water coil is leaking.
It could be the heat exchanger leaking, which will be lost as steam or flow down the condensate drain. Can you collect from the condensate drain over a period of time when the boiler isn’t running?
Does the system leak at the same rate if the system is running or if it is all turned off? If it leaks noticeably faster running, that could suggest the heat exchanger…
Running pipes in concrete floors can be ok, but they need to be wrapper. If not they will develop pin holes over time, and leaks will be very hard to spot until the leak is very bad. Can you fit a couple of valves to completely isolate the downstairs heating loop? If so, that can confirm if it’s that loop that is leaking, or not.
The leak is enough that I would expect to notice it, and that it wouldn’t just be masked by evaporation when it’s running. I’d guess at the boiler or in the concrete floor. When is the boiler service due? They should check the heat exchanger…
Good luck!