There was a comment on Gardener's World a few years ago, to the effect that it was useless to toss surplus snails into the neighbour's garden, owing to their strong homing sense. The presenter went on to say "if you don't believe me, try painting their shells with nail polish before you toss them!" I tried it by painting a snails shell and then carrying it down the garden - it took 5 days to return. Having painted others using several different colours and tried the same thing I would say a) they certainly live for many years as we still have the originals b) you can easily see them and so don't tread on them and c) a mossy wall after rain is like a field of jewels - try it if you don't believe me!