I have two issues that may or may not be related. Looking for advice from those who know about the rear drive unit. Bike has 40K on it.
1. My rear rotor seems warped. Been riding it like this for 2 K miles. No discoloration or sticking of the caliper is at all evident.
2. I noted some sign of gear oil last night on the rear wheel rim and for the first time on the rear disc.
Any ideas?

Oil might be blowing out the vent at the top. Check to see if that is the case. May be overfilled, if you've redone the fluids in the last little while.

I would say the two issues probably are not related.
A disc can and will warp for a myriad reasons that you will never find, especially the stainless types. I would replace it with an EBC floating disc which will be less likely to warp.
The drive may be a little over full as that can happen easily when filling if the wheel turns at the time it takes a lot of oil up the top of the case and when cold it takes a while to drain back and so it gets a bit overfilled.
The drives will at times let some oil escape and you may only be noticing this because you have inspected the disc a few times due to the warp. If you pressure wash the bike some water will get into the drive via the breather and when the drive heats up with use the water evaporates off and can take some oil back out the breather as the vapor escapes.
However all that does not mean there may be something wrong so I would change the oil and take a watch and see attitude for now.

This bike was new to me last year. I bought it as a project bike and am a seasons brick bike owner and mechanic. I reassembled the transmission, clutch replacement this last winter. I was careful in filling the final drive unit with 75-140 synthetic GL5 spec gear oil and it did not exceed the lower thread in the drive when full. I know what a SS or cast iron rotor looks like if it is overheated and becomes warped. This one looks like it has seen light duty. If the bearing checks out good and the seal is leaking. Can I just replace the O-ring seal?

I will check it again but I am concerned about it being related if the rear was serviced, as I suspect.

Has anyone ever needed to replace the vent?