It's supposedly in level 10 of Endless Paths in the southeast corner. I've used stealth mode and scouted the entire map, but it doesn't seem to be there. Anyone know where it is? I've looked everywhere in level 10, so I know it's not there.

How's your Mechanics skill?

 

Here is that unique arbalest. Try to raise your mechanics if nothing shows up.

Didn't realize mechanics had anything to do with ability to see hidden caches. Was able to find it once I raised my mechanics skill from 9 to 10. So needs a 10 in mechanics. Thanks for the help.

I am not sure why, but it took 11 for me. I had 10, did not see it, rested to get plus 1 bonus, got it. 

10 mechanic worked for me, with 15 perception, but with 8 mechanic I searched it and didn't find it, I had to rest to have +2 Mechanic (in Caed Nua) to see it ...

May be perception have something to do with discovering hidden things (as said in the perception attribute description, it permit to see things happening in the background (more or less, I translate from French))