I am clearing some old paving slabs which have been overgrown and covered with soil. I keep coming across patches of thin (<1mm) black material, looks like bitumen, with some kind of foil backing. It's possible they were stuck to the paving slabs (60cm square smooth concrete slabs) at some point, but if so they have delaminated. I also keep finding bits of broken horticultural glass in the same area which may or may not be related.
Any ideas what this might have been? My concern is whether it could be an ACM. Obviously testing is the way to be sure. But having some idea what it could be will help in working out where else I might find it.