If your mechanic can't find a major leak like that it's a pretty simple fix.

1. Fire your dodgy mechanic.

2. Get a new mechanic, explain the situation and ask him to fix your ride.

Coolant doesn't just dissapear like that and a simple pressure test will find it every time...without fail...unless your mechanic is an idiot or thief or both.

It pays to not employ this technique in small towns without serious thought first and it helps if your new mechanic is not mates with the old one.