Hi I work for a German manufacturer in the car trade and in order for you to Reject the bike the Dealer needs to be given enough attempts at fixing the fault.
If they haven't even found or replicated the fault you have no grounds for a rejection.
With experience I would try to replicate the fault sometimes this is difficult with electrical faults that sometimes can be intermittent, and or try another bike of similar age/model to see if it is just something that they all do, or go to another dealer and get a second opinion. This could even be an independent.
Anybody who rides a R12GS WC, won't appreciate the quality of a 1150GS..............they should just stick to riding the latest trinket, that makes them feel worthy and impresses their mates
Take the mechanic with you as a pillion ...I managed to induce some lovely popping sounds on over run.
I struggle to see how you can reject if they cant find a problem,try a different dealer to check the bike over.