Depends exactly why you went to the garage in the first place... If
you were low on coolant and they topped it up you probably had a leak
in the first place.

If you said "Brrrr, it's cold outside, could you put some antifreeze
in the car", they would have had to drain some coolant out of the
system to add the antifreeze and they might have had an airlock which
has just moved and caused your coolant level to drop enough to trigger
the light.

If the former, have a look for "damp" round the ends of the rubber
pipes, have a look under the water pump, and also get a torch and have
a look through the kidney grill and check for damp patches on the
radiator. Check the radiator when the car is dry and cold. When it's
hot, any water that leaks from the radiator core will just evaporate
off the hot surface.

Please state the year and model of your car in future posts as there
are known problems for certain models.

Dodgy.