I posted an image a few weeks ago showing a Morse code of mucus trails laid down by what I presumed to be introduced garden snails (Cantareus aspersus). I attributed the dashed lines to the snails because they were the only molluscs active at that time. (The slugs were keeping a rather lower profile then than they are now.) This led to a brief discussion about the source.
It's taken all that time for me to catch a snail in the act but I found one last night. Yep. The snails are responsible.
I knew you were waiting for the answer.