In Basic D&D, the default way to find a trap is to roll 1d6 and if you get 1 you find it.
But thieves have a special % of finding it: e.g., a first level thief has 10%.
Should we roll this % passively?
If not, should we roll both the d6 and the d% or just the d%?
From the manual:
Find or Remove Traps is a double ability. The thief has the listed chance of finding a trap (if there is one) and the same chance (if the trap is found) of removing it. Either attempt may only be tried once per trap.
While the default way is:
Any character has a 1 in 6 chance of finding a trap when searching for one in the correct area.
Best Answer
The reading of B/X leaves it open. In the period when it was for sale I can't remember a group that allowed a thief to roll twice. This is problematic at low levels because first and second level thieves have a lower probability of finding a trap than non-thieves. A 1 in 6 chance is 16.67% while first and second level thieves have a 10% and 15% chance respectively. For a character who is a specialist to be worse than non-specialists is problematic.
In the modern/OSR era the push for all characters to be able to try most things have revived the 1 in 6 which seems to have gone out of favor in the 3.x era (logically as it is handled by skills). In some cases these writers have argued that everyone has the 1 in 6 change to do thief skills (not just find/remove traps) but if a thief fails their skill is a second chance (or a saving throw). This is not the only or even most common interpretation from what I've seen but some OSR blogs champion it. One example is It's Okay, Gary Sent Us which is a revision of an earlier piece the author did for Fight On.
Doing some further research in my Dragon Archive CD, there is a Sage Advice answer in issue 62 about the chances of non-thieves having thieves skills and saying they do not. In general, the column was AD&D focused and without an explicit mention of B/X I don't consider it to overturn the 1 in 6 in an official manner. This point is emphasized by a question about contradictions between B/X and AD&D in issue 76 that said they were different and shouldn't be mixed. These were found searching Sage Advice in issues 52 (the introduction of B/X) through issue 80 (the end of 1983, the year Mentzer's boxed version of Basic came out). Based on the lack of an answer in Sage Advice in the relevant period I doubt TSR ever addressed the issue.