Calling all car gurus...Where oh where has my coolant gone?

A few weeks ago I noticed my coolant level was low. Filled up the tank and took it in for a checkup. My mechanic said I had a bolt on my timing cover that was leaking. ( Engine only has 63k on it ).
Drained the tank, repaired and sealed the bolt and refilled and ‘burped’ the system. 5 days later the resevoir was bone dry again, but no signs of overheating. Took it back last Friday and they found another hose somewhere with 2–3 ‘pinprick’ holes in it. Replaced hose, refilled with coolant. Today ( 5 days again ) tank is bone dry! Mechanc now says that it must be leaking ‘internally.’

Going back Thurs. Where could my coolant be leaking into?
I have no stains on my garage floor, it is like it is just evaporting witho8ut a trace! ??????