[Rev]

With the engine at rest, the fluid level in the tranny is above the junction of the dip stick tube and the trans case. The dip stick tube uses an O-ring to seal that junction. It can and does often leak at that O-ring seal when not running . While driving the trans fluid level is below that level and there's no leakage. A new O-ring seal at the dip stick tube/trans case might fix it if that's where the leak is.

Rev