I have a 95 650L and when I got it ,it also smoked for a few seconds when started. After a few rides it doesn't do it anymore.
The previous owner told me he didn't use it for months when I bought it.
I have no idea also what that was.
That was probably a case of dry guide seals from sitting. Once it was being ridden again, the seals got some lube and started sealing again.