As Luriant said, thieves don't trgger traps. You can put (using a chain/rope) an animal near your entrance to spot any thieves, they usualy leave once spotted.
Efficient trap layout is a bit tricky and depends heavily upon what you want to trap/defend. I' m definately not an expert on this but I'll try to give some examples of how I tend to use them.
Keep in mind:
- In all cases, the most important factor is that all units try to move through as little tiles as possible when moving from point A to point B.
- Enemies are more likely to flee in terror if they see someone getting killed than seeing someone getting injured. Some players might like it and is deffinately more useful if you don't have a good military to finsih off any enemy that survive the "mine field", but I prefer to use weaker* traps so I can soften the enemies without halting their assault completely. It also means that the traps themselves are less likely to jam because fewer enemies will die on them.
* Using 3-5 low quality weapons (and not trap components) instead of the maximum 10 per trap.
1) In narrow (1 tile wide) corridors with lots of turns, the enemy will (preferably) move diagonaly in every corner stepping only on the tiles I marked as "2" below. That, in turn means that any trap placed in the "1" tiles is unlikely to ever trigger.
w= wall
wwwww
w 1 2 <- Enemy comes from here
w 2 www
w 1 2 -> to fortress
w wwww
2) For staircases, the easiest way is to just trap all the surrounding tiles but it is a bit of an overkill. However, if you haven't assigned any traffic restrictions on your dwarves, you can observe which tiles they tend to use and emphasize these when placing traps.
x= up/down stairs, t = trap, w= wall
wwwwwwww w
w x t t t t t t t units avoid pathing through
w x t t t the middle tiles.
w x t t t t t t t
wwwwwwwww
wwwwwwww
w x t t Units prefer the lower tiles.
w x t t t
w x t t t t t t
wwwwwwww
3) "Herding" enemies to your entrance
e = fortress entrance, w= wall, t= trap
ee t t
ee t t
ee t t t t
www t t t
wwt t t t
wwwt t t
wwww t t
www t
enemy comes from here
4) For catching wild animals, you can either just place randomly traps around the map or if the map permits (large area on the same z level) it you can again observe where these animal move through and place traps around that area.