Coolant level going down very slowly

A4 2.8 1996 - 46,000Km

Coolant is slowly decreasing. About 1 month from max to min.
No leaks, smokes or smells.
Temperature - oil & coolant - Ok.

Curiously, when I topped the coolant to Max I noticed a few hours
later some coolant in the floor. (engine cold - not turned on in any
moment)

Can it be just a minimum fracture in the higher part of the reservoir
or should I be looking at somethng else?
I imagine that the coolant works under pressure - cannot understand
leaking with the engine cold and no pressure at all.
BTW: after the mentioned leak - no more than 12 hours later - no leaks
at all. (and reservoir at MAX level)
Thanks in advance for your help.
Jorge