How do we know that the nomads just pass through? Perhaps they notice the suspicious "artificial" cover.well really the question is Why are the Paynim riders there?
1 Are the Paynim just a random encounter passing through? - well the PCs have dealt with that by hiding so Outcome: the Paynim just pass through
2 The Paynim know/suspect the PCs are there and came to investigate - Outcome; the Paynim investigate
3 The Paynim have come to the Standing Stones to camp Outcome: the PCs are trapped and need to escape
How do we resolve any investigation the nomads undertake? As per my post just upthread, the relevant rules are defined in terms that make sense in Arneson/Gygax-style dungeon exploration - where space and time are closely tracked in a granular fashion - but don't work very well in this sort of context.