Can you post the model number please.
Also have you removed the cover to see the fan itself?
Ok theres nothing really inside the fan that cause this other then a small bit of oil. It may have been something got stuck at the back on the element and when it heated up it melted it and run down to the bottom.
As its brand new you may be best getting Bosch out to check it as if its stained the base you get them to replace the oven as I wouldn't want it like that plus they can remove the rear panel and check behind there as they may be more.
Or if you want to look yourself then the rear panel inside the oven is usually held on with 4 screws. Remove them and the rear panel will pull down and you can access behind there.
But I think from what you say something has got behind there an melted as theres nothing like goo inside a fan motor.