I assume your question implies that the coolant level drops below the low mark after operating the engine and the coolant system was full prior?
If the coolant has recently been replaced, perhaps the mechanic forgot to "burp" the system to remove all trapped air. Coolant then exits via the overflow tube during operation of the engine.
That occurred on my rig after a shop did the coolant change. After three months of engine shutdowns and top offs due to "low coolant" I bled the system and the problem stopped. (Use coolant bleeder valve and run front dash heater.)
Pull the oil dipstick and see if any coolant is present in the oil.
Otherwise, a pinhole leak in the plumbing could cause the coolant to disappear.
1995 CC Magna #5280
C8.3L 300hp Cummins, 31,000lbs