I think others are right in calling the disc OB, but where did you take the drop? I could see a case being made that the disc should take the drop in from where it had come to rest. If the disc had managed to land on top of some grass that was growing in from the edge, I would think it would be impossible to say whether or not some part of the disc had crossed into the in-bounds vertical plane right next to where it came to rest. It would seem like the benefit of the doubt would go to you and you'd take the OB right next to the disc, since it was so close...
This is the same idea as a disc thrown over water -- if it hits a treetrunk growing on the opposite bank, and falls into the water, then you take the drop by the treetrunk, since it crossed into bounds on the other side -- even if briefly. If your disc hit the grass on the edge of the sidewalk and nudged over the OB line, then ricocheted a few MMs fully back into the OB, then you'd get your drop there instead of where it initially crossed into OB off the drive.