Secret passages will be located on the roll of a 1 or a 2 (on a six-sided die) by men, dwarves or halflings. Elves will be able to locate them on a roll of 1-4. At the referee's option, Elves may be allowed the chance to sense any secret door they pass, a 1 or 2 indicating that they become aware that something is there.As the player of Dordagdonar in my Dwimmermount campaign will happily tell you, I keep forgetting that, in OD&D, an elf's simply passing by a secret door merits a roll by the referee to determine if they sense its presence. But I'm even more forgetful of the fact that an elf, if actively looking for such a door, can find one on a roll of 1-4 on 1d6. Indeed, I'm so "forgetful" of this fact that i don't think I've ever used the rule.
I fear this is a case where my having played AD&D for some long in my younger days made me forget something I should have remembered from Holmes (which, as the Blue Book usually does, preserves a rule from LBBs). In AD&D, elven passive perception of secret doors is reduced to 1 in 6, while active perception is reduced to 1-2 in 6 (except for concealed portals, which is 3 in 6 -- can anyone explain how a "portal" differs from a "door" in this case?). Interestingly, Moldvay/Cook does not follow OD&D in its treatment of elves and secret doors, presenting instead something akin to AD&D (1-2 on 1d6 chance) but without any explicit provision for the passive perception of secret doors.
Anyway, I'm going to try very hard to keep these rules in mind in the future. It's one of those things I periodically remember and then forget again and I always feel bad about it afterward, since there is an elf in our campaign and I'm sure the party has missed more than its share of secret doors because I can't seem to get this simple rule through my head.