The blood of a thief runs through your veins. With the Thief Perk, you get a one-time bonus of +10% to your Sneak, Lockpick, Steal, and Traps skills. A well rounded thief is a live thief.”— in-game description, Fallout 2
Fallout 2 and Fallout Tactics
+10% to your Lockpick, Find Traps/Secret Doors and Sneak skills.