It doesn’t matter if it was pre-planned or on the fly. Unless you want to make the claim that the footprints are there solely as a threat and can never act lead to anything more? I’ve not read Stonetop but I don’t think that’s the case, if only because it would lead to players not caring about threats once they noticed a pattern.
Telegraphing a threat is meant to lead to a decision point, especially in an exploration context. I want play to move forward, so I’m prompting them to perhaps Know Things, or Seek Insight, or simply declare a course of action and evolve the situation.
Just because a Threat comes on-screen doesn’t mean that I’m proscribing any future actions either. Last time we played I provided signs of a threat (dead spots of grass, signs of strange animals), on a Seek Insight 6- I Introduced a Danger by having the wolf-deer chimeras closing in with their horrible elk-bugle/howl call, the players did some back and forth and elected to creep out using a magical ability they’d already had in effect.
With some more back and forth and development of signs of a larger problem in the forest behind these things, we left the ruined tower hill behind and the scene concluded.
Did this become an encounter because the threat manifested? Was it an encounter when it was simply a ruin on a hill and a handful of notes of potentiality? Jeremy Sandberg intends exploration in Stonetop to be sort of a “narrativist point crawl.”