The first book of gaming goodness that I ever owned, months before I ever played in a roleplaying game or really even knew what RPG meant, was the AD&D monster manual , 4th edition, which I have sitting on my lap now.
Many of the stats listed for the beasties seemed cryptic to my 11 year old brain, but perhaps none more so than % in Lair.
I think I, along with a great deal of other gamers thought this little stat meant “How often the monster is home”, and that seemed pretty useless considering the game was supposed (I thought) to revolve around detailed adventure modules in which the designer choose what room or other a particular monster was in.
% in Lair was thus safely ignored for 30 years.
And then I read this little bit in the FFC. “For each encounter, consult the Encounter Matrix for the type of creature that lives at each spot… The normal chances of the creature being in it's lair are worked out as they normally are. So if Encounter Six has a 30% chance of being found in it's lair, then that percentage is used and the number of creatures encountered will then be any number up to the total number present in the hex. Again to avoid confusion, you may wish to take the maximum number of that are listed on the Monster Matrix as representative of the population in the hex for each encounter, given a plus or minus 10% to keep the players on their toes. For each time that the creatures are found in their lairs, there will be a chance that a portion of them are out in the countryside. To determine this number, assume that 40% of the population is always in the camp and that up to 60% (10 - 6O%) are always outside of the camp.”
Wait, what? Here Arneson is saying percent in lair is not the chance that a creature will be home – that chance he gives as 40 – 90% after being found “in lair”. hmmm. Next I turned to my trusted MM in the hope of clarification and read
“% in Lair indicates the chance of encountering the monster where it domiciles…..”
So % in lair, whatever else it might be, it indicates the chance that players have actually found a lair in a given hex, be it a dungeon, a castle, a thieves den, whatever. It turns out to be a vital statistic for hexcrawling.
Gygax, in the MM goes on to indicate that it will also mean encountering the monster. But he seems to be missing something here, the fact that not all monsters are singular and multiple type monster populations – bandits lets say, aren’t always all in the same place at the same time. It would seem that % in Lair was one of Arnesons stats for hexcrawling that Gary may not have fully used or understood in the same way, leaving out the % not home part. So when we turn to the FFC and read 40 – 90% of a given population may be out of lair when the lair is encountered – it becomes clear that the "in Lair" stats most important function, one very useful to the hexcrawl Referee is telling what the likelihood of randomly finding a lair is, regardless of how many or who may be home.
For single creatures, it gets interesting. Dragons especially so. The FFC, gives different chances of a dragon being in its lair depending on type and gender and it does so in two different and conflicting entries – one co authored with Richard Snider, the other, older d6/CHAINMAIL based material. Arneson and Snider’s AiF also gives a % chance a dragon is home that differs yet again. None of these are anything like OD&D’s % in Lair stat for dragons. So in the case of dragons, it’s easy enough to use one of Arneson’s stats for determining when various types of dragons are in their lair, and OD&D’s % in Lair for when the lair is found itself.
In summary, a % lair roll should be made after a positive encounter roll occurs when adventurers enter an area to see if they have stumbled on the lair or some wandering group outside their lair. if it is a wandering group, it will be some portion of 10 – 60% (1-6 on a d6, split in half for each roll of 6) of the lair population.