That looks like an oil leak from the rear shock absorber.
The BMW dealer picked up on the steering bearing but not the rear shock. If all you've done it trailer it home, I'd be going back to the dealer. Just don't tell them it's been bounced off a trailer.
In the UK, a BMW Approved Used Bike would be sold with a 2 year warranty. Having said that, the shock absorbers may only be covered up to 30,000 miles. That is the case with the extended Insured Warranty.