When you might add fluid, are you doing in when the powertrain is fully warmed up? Correct methodology and such, too? It could well be that the trans might be getting over-filled and is just getting rid of the extra fluid it does not need.
By observation, transmissions do not periodically lose fluid, but lose it steadily instead.
It is very possible that the front pump lip seal (seals the converter snout to the front pump body) is leaking, but it will leak continuously until it is replaced. Sometimes, that lip seal will wear a shallow groove in the converter snout, so a new seal in the same place might not fully fix that issue, without a fresh converter.
IF the trans only leaks after the car has sat for a good while, that would mean the atf level in the case has risen above where it would normally be. Any seals BELOW that level could be suspect, including the neutral safety switch on the lh side of the trans.
Just some thoughts and experiences,
CBODY67