Last week I noticed this odd track made of "squiggly triangle footprints" on the outside surface of a dirty skylight in my home in the northern Netherlands, and I found it odd that I can't imagine what kind of animal this track would have come from. I assume it's animal since the movement seems very intentional and the triangle shapes are periodic, rather than being a continuous smudge that might be caused by a tiny leaf blown in the wind for example. Each tiny triangular "footprint" is around 3mm wide. Any pointers on how to identify what caused this track?
It is really obvious if you have seen it before. It is the track left by a feeding snail.
Here is the grazing pattern of the common snail.