Here’s what Moldvay has to say about the disarming traps thief ability:
remove small traps (such as poison needles)
However, on page 22 when discussing traps as part of the adventure:
Any character has a 1 in 6 chance of finding a trap when searching for one in the correct area. Any dwarf has a 2 in 6 chance (This does not apply to magical traps such as a sleep trap.
What’s interesting here is the thief is listed as having both find and remove traps (with the same score) which begins at 10%. No details are given for the find part.
While OD&D doesn’t give any numbers on finding traps that I can find it does have the same verbiage on springing traps (1 in 2) as Moldvay. Right before that section in Book 3 is the discussion on finding secret doors. It seems Moldvay’s trap finding is just an application of those rules with dwarves instead of elves getting the demi-human advantage.
Moldvay’s decription of disarming traps matches Greyhawk and Holmes pretty much word for word and both Greyhawk and Holmes do not mention finding traps, just removing them.
While Moldvay gives not description of the find part of the skill the AD&D Player’s Handbook gives us the most detail of any early (1980 or earlier) set:
Finding/removing traps pertains to relatively small mechanical devices such as poisoned needles, spring blades,and the like. Finding is accomplished by inspection, and they are nullified by mechanical removal or by being renderedharmless.
This verbiage is pretty much intact in 2nd Edition AD&D with some additions:
The thief is trained to find small traps and alarms. These include poisoned needles, spring blades, deadly gases, and warning bells. This skill is not effective for finding deadfall ceilings, crushing walls, or other large, mechanical traps.
The addition of gases and spring blades make it a little more vague on where the line is even if an outer limit is set.
What I make from all of this is the thief’s traps abilities are relatively weak. If you’re allowing a thief to find pits, falling blocks, deadman’s crossbows, or similar large scale traps with a simple role you’ve expanded this skill beyond what was intended. The same if you let him disarm such traps. Now, I’ll admit that’s how I long played and my experience with D&D3 and later seem to formalize that in their find and disable device skills.
However, if we unlearn what was in later additions and go back and read the thief the traps ability is a narrow one. Clearly, other characters can find large architectural traps and play skill is still needed to disarm them. That’s probably a good dividing line: anything part of the dungeon or other structure is not a trap accessible to thief skills. A stand alone object the thief can pick up and move or simple mechanisms mounted to walls is accessible.