Loosing coolant, not on ground under car. on 1999 Cadillac Catera

I have recently had system flushed and pressure tested. No oil in water reservoir and no water in the oil.
Uses coolant a lot faster with the A/C on.

Asked by for the 1999 Cadillac Catera
no tsb's on this,but there are some about alum heater cores and radiators. poss water pump leaking only while driving?