I am guessing your Carezza is a boiler (rather than thermoblock) machine? If so, these machines usually have a safety (pressure relief) valve built into the middle / bottom of the group/boiler assembly.
They are nothing more than a spring and ball recessed into a channel in the group, just above the shower.
These things leak commonly enough however it is usually manifested when you are in steam mode, not usually when you are "only" in coffee mode, as there is not enough pressure in the boiler at that time to push against the spring tension and open the valve.
Are you leaving the machine to idle indefinitely in steam mode, or is this leakage happening when you are in coffee mode?
If it is indeed occuring when you are "only" in coffee mode, then I would suggest
a) the spring tension in this safety valve is fatigued OR,
b) the ball (usually a plastic bead) could be worn where it seats OR
c) there could be a piece of grit lodged in the seat of the valve OR
d) as the boiler / group assembly is aluminium, there could be a bit of corrosion on the seat
The usual course of action would be to remove the ball and spring and inspect, give the orifice / seat a clean up and refit the same ball and spring and test. If this does not solve the problem you could replace the ball and spring with one of exactly the same type (obtained from Gaggia repair / service) agent. Use only the "genuine" part or you risk the valve not opening when it really should, turning this into a real safety issue.