This sounds like a very suspicious diagnosis to me.  The mechanic claims
that he can see the leak?  But you do not have any coolant on the ground or
lose any coolant from the system?  Sounds like a scam to me.  Since he
claims that he can see the leak, it would have to be external.  Run the
engine fully up to temperature (in park) and then leave it over clean
pavement and double check for coolant drips.  Otherwise, just be very aware
of the coolant level in the overflow tank and watch for any decrease.  If
you are uneasy, you should get a second opinion from a trusted mechanic.  I
would not pay the guy for a job that big just on his "say so".

There are tests that can be performed such as exhaust gas analysis and oil
testing to see if the coolant is leaking internally.  But then if that were
the case the dealer's mechanic would certainly not be able to see it from
the outside...

Another thing, why did he replace the (expensive) fan clutch?  You didn't
have symptoms of a bad fan clutch.  Bad fan clutches cause overheating at a
stand-still, not noises.  It sounds like he may have already padded his bill
once with you.  You probably only needed a waterpump, and it was agood idea
to do the thermostat at the same time, but fan clutch?  That's not a
Preventive Maintenance item...

-Fred W