Id think a magic finger...I think that bolt is aluminum so a magnet won't help - to find the 'threaded piece' that is missing. But you may end up dropping the oil pan. It would be damned near impossible to take back out. So you may want to cut your losses, remove the oil pan and get it out of there...No way to leave in in there and risk the damage it could create for you.
99 540i-6 Sport & 07 Z4///M Coupe