Check your drained oil carefully. If it is not in the drained oil; and it is most likely that it is not there; then it is in your oil pan.
It is simply too risky to drive around with a broken bolt stem in your oil pan. One bounce over a pothole and the piece could jam itself into any part of the reciprocating parts of the crank, rods, pistons, etc. If that happens, your engine will be severely damaged...like a hand grenade. I wouldn't even crank the motor over with the starter.
First attempt to capture the bolt with a magnetic tipped flex shaft. If you can drag it to the oil drain plug hole, there is the slight chance that the length of the bolt stem is shorter than the diameter of the drain hole and you can pull it out.
However, I doubt that you will be able to get it out without pulling the pan.
Any other ideas? gordo