Audi Repair: Audi A4 coolant leak, coolant lines, coolant system

Question
I recently had the engine replaced in my 2003 A4.  Since the replacement with a Audi rebuilt engine I have been loosing coolant.  The first time was about a day after I picked the car up.  They told me that the new engine burned it when I used the heater.  Then about a month later the reserve tank was empty again.  They told me that it was a seal.  Three months later the reserve tank is down about a half an inch in coolant.  This time they could not find a leak but told me that it was nothing to worry about and that some Audi engine use coolant and as long as it was not a drastic loss there would be no harm.  What should I do.  All work was done at the Audi Dealer.

Answer
Considering Audi Guarantees their coolant for 'life' then you definitely have a problem, either the system was not bled properly when refilling the coolant or they have missed a leak, have the coolant system pressure tested ( when cold) and see if the pressure drops, if it does look for the leak which will be more obvious with the pressure tester connected.( hissing noises for minute leaks or coolant spraying out)  also look for rust like residue along the coolant lines which would indicate leaking coolant, do not ignore the passenger compartment when performing these tests, If no leak is found then check your coolant cap, it should 'click' when tightened fully. replace it if in doubt.

from experience i know that the flange on the rear side of the engine( side near the battery) that holds the 4 pin temp switch usually gives out so it might be a good idea to start there.

let me know how it goes