think I spotted some Red Nail Gall Mite (Eriophyes tiliae) . not 100% sure on the ID there, but the shape is right and the leaves have fallen from a common lime / linden Tilia x europaea which , as you can guess from the name E. tiliae , is the host of this mite.
The galls form around the feeding, growing mites. Once winter comes the mites shelter in the bark of the tree and wait for the leaves to re-grow. as far as we know the galls are harmless.