This part was so damn annoying...The best advice I can give is as follows, but even then it is some guesswork. There is no marker I can even point you towards to help ;_;
It is definitely to the right of Catherine as the clue says (psh big help, right??) No not really. Stupid useless hint!
Anyway, so it's on that wall. The dark one...yay
It is at about eye level...And it is close to the spots where Grissom says: "nice job being thorough." It is in pitch blackness. Just keep poking around in the said area and you will -eventually- find it. There is no way you're going to see the first aid kit that it is in, you'll just see the hotspot light up yellow saying investigate closer, zero in and then you'll see that you gotta make use of the high power flashlight.
One of my biggest complaints about this game is the ridiculous dark areas. If they are gonna give you them, then fine, but make the flashlight accessible at all times ):