In the original rules, travel through the wilderness calls for a check for wandering monsters once “at the end of each day”, with the odd caveat that seaborne encounters will occur in the middle of the day.
So you roll the dice. If you get a monster you check the % in Lair to see if our intrepid adventurers have stumbled on a lair or encountered some non lair or out of lair troublemakers.
That’s a workable method. The trouble is that it lacks any granularity with regards to the map. What I mean is that characters may be traveling 3, 4 or more hexes in a day. So where does this encounter take place?
If it is truly at the end of the day, presumably as characters prepare to camp or lodge for the night, then it will obviously be in the last hex traveled in. If on the other hand the encounter occurs at midday, then one of the other hexes must be picked and travel halted at that point.
Referee’s can do this. They can pick a hex, either randomly or deliberately for the encounter, and again this is a workable method; But it is not a very sharp method.
When I first started to work on the “evasion” table, it was less than clear to me what exactly it was intended to do. One description of it – I think it was in AiF – said it was “the chance to avoid an encounter”. This set me to thinking, as they say. While I think the major intent of the evasion table was originally for hexcrawl chases in the Wilderness, there was a flexibility there (as with almost everything in OD&D) to apply it to other “surprise” situations.
Now if the chance of party A to avoid party B is 70%, then conversely, the chance of Party B to find party A is 30%.
Viola! The Chance of Discovery. Rather than rolling once a day regardless of how far a group has traveled and backtracking to figure out where an encounter may be, Champions of ZED gives the Referee the option of checking each hex as the PC’s pass through. For each new hex entered, a roll is first made to check for lair encounters using % in Lair, followed by roll(s) on the Chance of Discovery column made for all “wandering” groups in the area. If an encounter is indicated, the Referee then check surprise.
Using the method is optional, of course, but it is an example of how the original rules can be stretched a bit when needed to cover more than the obvious situation and give a little more granularity to hex travel.