I have a fair bit of ATF leakage from my PCM 1.23:1 transmission and have read the threads on the possible points of leakage.  Mine seems to be leaking when not running -AND while running most likely, but it certainly drips while sitting in the garage between use (my boat gets used maybe 2x/month).  Does the fact that it leaks while not is use rule-out or give any clues as to what is the most likely leak point?  It probably drips about 1-2oz of fluid while sitting for a week.

The front seal leaks into the bellhousing area and can puddle up and leak after the engine is off.

The rear seal usually drips pretty much straight down just in front of the output coupling and the oil level is probably high enough for it to leak when at rest too.

You could also have a hose leak or a fitting leak where the hose goes into the transmission.or into the cooler.

You have 2 hoses, the supply to the cooler and the return from the cooler. check them for leakage.

Spread some paper towels or whatever you want under the transmission to get an idea where the leak is dripping from when it's in the garage

The leak may stay the same for a long time or it could get worse,,,,,,,,,,,,,,,great answer huh?

If you monitor it regularly, you'll know what it's doing.

Fixing it involves pulling the transmission and replacing the front pump seal if it's the seal. There are also some small O rings on a PCM that could be leaking but you can't access them until the transmission is removed either.

if you go to the link below you'll find a guideline for removing a B/W transmission from an earlier model SN.

You have a PCM transmission but all the basic steps are pretty much the same so it'll give you an idea what's involved 

It dealt with the replacement of the damper plate while the transmission was off and with yours being 21 years old, it's probably worth considering. 

The PCM 40 series takes it's own "special" damper plate that's different than a plate for a B/W transmission That's secret code for more expensive