Looks like an anchor piece (one of two) that attaches to the top of the machine (bracket at the right end) and then anchors via a screw (holes at the left end) into the underside of the countertop. It's exhibiting some rusting so is unlikely to be from the new Bosch (unless the installers re-used the anchors from your previous dishwasher and one has fallen off). Or it got left behind and fell into the Bosch and you just now noticed it. Some models anchor to the cabinetry via other methods.

Yes, it attaches the dishwasher to the underneath of the countertop.

Or to the side of the cabinets