Could a flat tyre two days later be due to a clumsy service?
To me, this sounds like coincidence. If the shop had punctured the tube and caused a leak, I don't think it would have held air for two days before showing any signs of going flat. Especially given the fact that you reinflated the tube and it went flat again by lunchtime. The leak is fast enough to make the tube go flat in just a few hours, so it seems unlikely that the tube would be OK for Tuesday and Wednesday and only start leaking on Thursday. When replacing the tube, you should look for the cause of the leak. You might be able to find a foreign object that caused it, but it's possible you won't with a typical puncture. If you find the side of the tube leaking, that might indicate that it got pinched by the tire during installation