First off you opened the reservoir while the car was hot, thats a no no. Second, well of course the fluid will come back to the line its hot it expands! I can't believe the mechanic agreed with you. Check it while its cold, it doesnt sound like its too low though but top it off anyways.

 

yea be careful if your coolant is low and your engine is hot when you unscrew the cap it could come back up into the reservoir really fast and even overflow and/or spray. You don't want that. If your engine is hot and you unscrew the cap and the fluid level raises, let it sit for a while, it will gradually go back down... then fill it up from there. But you shouldn't have to do this unless you have a leak somewhere.