Boiler..low pressure

Fill up with water until pressure is at 1.4 bar

Check boiler first to not lose presure

Chec the air pressure in the expansion tank, shouldn't be around 3 bar
Because the water pressure is higher then the expansion tank it allows the water to go on over pressure.

A leak in the heating system is usually the cause. The combi boilers usually function with a 0.7 to 1.7 bars pressure and if it goes down then there should be a leak in the system.

Increase the pressure with filling loop

Pressurised systems work on pressure, so even if your heatings turned off, if it drops pressure you loose hot water and heating, so you've either got a leak on your heating system or a problem with your expansion vessel.

Leakage in central heating pipes is common and can be caused by a number of issues.
In this case the pipes in your floor could be deterating due to pipes not insulated, joints worn, or possiable from the inside out (copper pipes) stagnet water sitting in the pipe work - central heating plumbing contains a circuit of water which when heating is on , the water flow rate is moving. But when the heating is off ,the water is sagnet. And if is not purefied, the pipe work can decay from inside of the pipe out casuing leaks.

Boiler expansion vessel faulty