That system is analogous to the Stealth check in 5e D&D to avoid an encounter roll. It doesn't resolve hiding from nomads who have turned up by conjuring a cover over one's pit.
Here is the relevant text (p 134 of my copy of C&CL, and it is also found on p 149 of the RMSS GM Law):
1) Avoidance roll — the PC group makes a “avoidance” dice roll and applies the appropriate modifiers to get an adjusted result;
2) Activity roll — the GM then makes an “activity” roll and applies the proper modifiers to get an adjusted result;
3) Roll comparison — the GM compares the two adjusted rolls;
4) Negative differential — if the avoidance roll exceeds the activity roll there is a negative differential and no chance of an encounter and the table is not used, and any pursuit is thrown off or avoided. This may be modified due to an unusual circumstance, or if there is a tracker in the pursuing group.
5) Positive differential — if the activity roll exceeds the avoidance roll there is a positive differential and the Standard Encounter Chart is consulted.
The Avoidance Roll modifiers include adjustments for speed and mode of travel (going more quickly makes you more likely to be noticed; the adjustment for flying and for sailing is listed as "variable"), for night time (this makes you less likely to be noticed) and for camping (this makes you more likely to be noticed). The adjustment for "Spells employed" is "variable".
The Activity Roll modifiers include terrain and weather adjustments (the clearer these are, the harder it is to avoid an encounter), literal activity adjustments (eg +100 to the roll if a hue and cry has been raised), and adjustments for night time that are sensitive to the difference in this respect between humans and Orcs. For "Spells", the adjustment is "variable".
That system is not useful for resolving the scene I described in the OP.