If the trans is not overfull, then T/C drainback (if it is draining back) should not bring the fluid high enough to leak out the vent.
If you are positive there is no leak from the front seal/Bellhousing area....then...
Possible souce of leaks after the car sits for awhile...
In no particular order...
1. Fill tube O-ring
2. Servo cover gasket
3. Pan gasket
4. Extension housing gasket
5. Modulator gasket or O-ring (depends on year of trans if push-in mod or screw-in mod)
6.Speedo cable O-ring
7. Rear seal
8. Shifter and KD seals.
All of these things can leak and the fluid runs down to the pan rail and "runs around" the pan gasket and "looks like" a pan gasket leak.
My bet would be the Fill tube O-ring or servo cover.
Sometimes I have had a C6 that "refused" to stop leaking at the servo cover. The souce was traced to either a cracked cover or a warped gasket surface. In the case of the warp, installing double gaskets sometimes works to stop the leak.
All automatics "drainback" to some extent, and slightly raise the fluid level in the pan.
Normal running level is right at the level of the pan gasket rail.
Slight drainback raises the level to the point where the servo cover gasket and Fill tube O-ring are.
Those are the 2 main suspects. Shifter and KD seals are also common leaks.
Check the fluid level after the car has been sitting and before starting the engine to see how high the level is after sitting.
I bet you see the level will be approx an inch or 2 above the full mark. That is not high enough to leak out the vent, but plenty high for the fill tube and servo to leak.
If you don't mind spraying some paint on the trans, try this...
Clean the trans really good with brake cleaner in a spray can. Get it good and dry with the brake clean. Blow off with compressed air. Then paint all suspect areas with some silver or "aluminum" color paint.
The leak will show instantly as a track in the paint.
Hope that helps.