Transmission leak advise

My first guess would be the O ring for the dipstick tube but the best way to be sure is to clean everything up really nice and make sure it is dry then take it out for a spin but try to avoid the freeway so you don't blow the fluid all over the place. If it is really dripping as much as you say, you should be able to notice a small streak after a good heat cycle. I've also had it happen where leaks don't start leaking until the parts begin to cool down so I would check as soon as you can after the drive, and if you don't find anything, check again in a couple hours once the parts have cooled down.
Good luck