I was ready to write this off as early humans ancestors depicting basic shapes of objects in their surroundings, until I got to the upside down and right side up “brush shapes”. I’m not sure what that would mean, something descending and ascending with some type of force or vitesse?

A lot of the signs can be seen as symbols that are easy to draw with the hands. For example, there's the 5-dots and 6-dots symbols. Those are: "every finger of one hand" and "three fingers from each hand". The brush shapes are "bring all fingers together". There may very well be a communicative aspect to the symbols, but I think they were derived from decorative primitives that are kinesiologically-determined.

That would help account for consistency over place and time.

But meaning? I'm guessing something like "# was here".

Possibly ad-hoc way finding, and reference points when lost, to help detect if you're going in circles, or if a change of direction is necessary.

If there are N number of confusing junctions, it would require the same number of markings as a unique mnemonic device to register the steps to escape the confusing area, and jog one's memory of how directions were negotiated the last time.

Or perhaps locations of good sources of food, shelter and so forth

Ancient hobo signs makes a lot of sense, wonder which one says "Tell pitiful story for food."