In Labyrinth Lord, Thief skills reach maximum values at level 14. Even though these are good skill levels, the cap makes Thieves unappealing characters at high levels, since all they gain per level after this point is a mere one Hit Point per level, a few more rows on the Attack Table, and a single improvement to their mediocre saving throws at 17. For this, they need to earn 120,000 XP per level past name level, which is not a fast rate (it's equal to Fighters, and the Cleric needs only 100,000 per level).
Frank Mentzer's BECMI edition dealt with the maxed-Thief problem by retroactively nerfing Thief skills in the Companion Set so they actually needed more levels to get 99%+ abilities. I think that Thieves are underpowered as it is (hence my house rule to up their HD to d6), so that's not a solution I favour. Apparently (I'm going from second hand information, since I've never owned this one) the Moldvay/Cook Expert Set intended a different approach, promising unique advanced skills for higher level Thieves in the Companion Set (which never got made). In that spirit (and also inspired by the AD&D 2e High Level Campaigns book) I've had a go at making up Master Thief Skills.
MASTER THIEF SKILLS
At level 15 and up, a Thief gains 15% to two separate Master Thief Skills per level. The maximum that any of these skills can be raised to is 75%.
Master Thief Skills List:
Hide in Plain Sight (advanced Hide in Shadows)
This skill allows the Thief to attempt to Hide in Shadows even when he is already being watched. In combat, a successful Hide in Plain Sight can set up a Backstab for the following round.
Swift Glide (advanced Move Silently)
This skill allows the Thief to Move Silently at full movement speed, even running. A Swift Glide roll allows passage across liquid surfaces, requiring a walking pace (no slower or faster) and no other actions; failure means falling in.
Jury-Rig Traps (advanced Find & Remove Traps)
This skill allows the Thief to reset a trap he has already disabled with a trigger of his choosing, allowing the possibility of tricking dungeon denizens into their own traps. The skill can even be used to move a trap (unless it's on a large enough scale that this is impossible) or disassemble parts for the Thief's own purposes (for instance, to retrieve an intact bottle of acid from an acid trap, replace it with holy water, etc.). Failure at this skill doesn't result in triggering the trap, it simply means that the operation is beyond the Thief's abilities.
Finesse Locks (advanced Open Locks)
This skill allows the Thief to pick a lock without tools, or to pick a lock with tools in a single round. If the skill roll fails, a normal Open Locks roll can still be attempted (with tools) on the same lock. Another use for this skill is, after picking a lock, to sabotage it so that the key(s) it was made for no longer fit, but the Thief can open the lock by Finesse without having to roll.
Wallrunning (advanced Climb Walls)
This skill allows the Thief to climb at the same speed as moving across level ground, and still keep one hand free. A failed skill roll means that this advanced form of climbing won't work in this case, but it doesn't mean falling, and a normal Climb Walls can still be attempted. Should the Thief fall (for any reason) within reach of handholds, he can attempt this skill to grab on and prevent the fall; if he falls without anything to grab, a skill check still allows him to take half damage.
Unfasten (advanced Pick Pockets)
This skill allows the Thief to unfasten armour or clothing on a target by undoing buckles or clasps, cutting straps, etc. This can be attempted in or out of combat, with chances modified as per Pick Pockets. Success allows the Thief to unfasten the target's apparel either immediately, after 1d6 rounds, or after 1d6 turns (Thief's choice from the three options). The effect is to halve movement, and reduce the AC of armour by 2 (and possibly reduce the target's modesty!). Multiple unfastens increase the AC penalty but don't hinder movement any further. The target can cancel the movement penalty by spending one round sorting his apparel, but the AC penalty can only be sorted by spending a full turn putting the armour back together.