I am a trained aeronautical engineer, and pilot. i can a interest in ufos and am very familiar with information on youtube. i was waiting for the rolling stones concert to start. i noticed this grey orb passing above and abeam the stadtpark . it was definitely not a balloon, bag, uav, drone – it was definitely a orb. i have never seen personally such a object (except on youtube) its showed no interest in the event. it maintained height, but appeared to drift with the wind as if a balloon (but was not a balloon). i pointed it out to my friend. no one seemed interested – as it was simply a orb. i maintained visual contact until it was too far to see. theoretically 100,000 witnesses (because of rolling stones concert) – but it was boring and uninteresting to look at.