I always slip my Olympus Tough camera into my pocket when I set out to work in the garden and, even before I’d started repotting plants in the greenhouse, I noticed these zig-zag patterns on the seed tray I was using.
They look like the marks left by a snail scraping away a film of algae from the surface of the tray.
Probably one of the garden snails that I’ve evicted from the greenhouse on several occasions.