In Basic D&D, the default way to find a trap is to roll 1d6 and if you get 1 you find it. But the thieves have a special % of finding it: e.g., the first lvl 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.

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.