There are other hoses that carry coolant as well -
Small hoses under the intake manifold and the heater hoses.

If you were not having trouble before the mechanic played with it, it really does sound like the mechanic has not put something back right.
Tooo much of a coincidence otherwise.

The other option has also been mentioned above - if the mechanic didn't fill it and run it, then top it up, you could have had an airlock that caused your original overheating.
After that overheating, it could have developed a leak anywhere.
If you saw coolant on top of the motor, there are a couple of small hoses hidden under the intake manifold, and also some o-ring seals that could have been damaged by the overheating.

Hard to tell what is wrong without seeing it.
It might be worth going to a cooling specialist and getting a pressure test done. That will prove if it is leaking, and give you a good chance of finding out where.

Good luck.

PK