Small pieces of ??? coming out of the door

There are blackish colored rubber? pieces that look like wore out tire tread, very brittle, falling on the floor under the door. Today we found some inside the dishwasher under the vent in the door. The washer hasn't shown any signs of leakage yet...but if it's some sort of seal, we'll need to have it replaced. Should we take the door apart? The manual doesn't show anything that looks like whats coming out. kenmore elite dishwasher 665.16263402