Engine Coolant Expansion overflowing?

My car is 2003 Ford Fusion with 59k miles on it, I have had it a year and so far had no issues what so ever with it.

Today it went in for its MOT, and I got a call from the garage to say they had aborted the MOT because while it was having the emissions test done it started randomly spitting out loads of smoke from the car and exhaust so they had to stop the test.

They said without doing a full diagnostic they have no idea but suspect piston rings or head gasket problem.

Issue is that this seems either very unfortunate and coincidental or something went wrong at the garage.

Drove it home from garage slowly car drove fine and no apparent smoke, left it to run for another 5 mins and still nothing. (worth noting that there's a burnt smell as soon as you get in the car now)

opened bonnet to look at oil and coolant and found that the coolant bottle was half what it was just a couple of days ago. There was also pink coolant lying on the top of the bottle and around the engine bay so it had clearly overflown at some point during mot.

Any suggestions on what could cause such a sudden overflow of coolant? Wondering if during the mot the engineer has either forgotten to put the lid back on or has put it on cross threaded which would have allowed it to overflow. Or it could be an issue with the car but what?

Will take it to another garage for them to look over it but feel I need to give them something to go off otherwise they could be looking in the wrong place for issues

The head gasket is likely blown. Confirm with a head gasket chemical analyzer.

They could have done any kind or sabotage, or the thermostat could have gotten stuck, or the electric fan could have stopped working. Refill the coolant after the engine gets cold, then check the level daily while still cold. That way you can tell if coolant is getting lost instead of the first shop trying to create a bad situation they can make you pay for.

Yes. Bad thermostat.