Sorry, that it ruined your experience. It should be not so hard on the first level, and nobody before in comments doesn't report it.
Some tips that you could miss:
- When the thief appears, your light spot is making short blackout. It means in some room the thief is presenting.
- Even if you in the room with the thief, it could hide in the nearest box when you getting near them. After you leave the room on some long distance the thief will leave the hide box too and try to steal the treasure.
- If you know that some one in the room, you can fly away for a few secs and then return to it and try to catch the thief when they move out for a long distance from the hideout.
- When you very close to the room when the thief is hiding you can hear his footsteps if he doesn't hide in the box.
So, the main goal to patrol the rooms and try to find the thieves when they far from the hideout.