The spot where I found some empty shells of the snail Neohelix albolabris last Tuesday yielded more shells of the same species today.
A couple of the shells had their periostracum more or less intact (for example, the one on the right in the picture above), indicating that the snails had died recently. So live snails are likely to be around.
I intend to go back there soon and do an intensive search to collect all the empty Neohelix shells I can find. Updates may follow.