//// An airlock in the circuit? Faulty sensor? ////
My first thought was an airlock as well.
There's probably a bleed valve somewhere ...... a competent garage should have bled it properly in the first place :(
In the days when changing the antifreeze was an annual event, I used to "burp" mine by squeezing the top hose. Put some more water in, burp again until it's just water squishing.
If it is a BMW, what year? the Bleed screw's is just on top of the top hose, and another just behind that, it's the size of a 5p with a X on it to insert a Flat head screw driver, but be advised these cars can be a right pain to bleed & prone to Head Gaskets should the Tem Gauge get to red.