just did this one, it's behind one of those movable blocks you jump from, it emits a pretty obvious light coming from the hole after the moving box moves
I ended up looking it up in that unendurable five minutes before you replied, but thanks. :)
Related:
The chapter menu makes it look like it's in the second part, and I looked all over there multiple times, but it's after the place that you start the third part if you select it. Boy, oh, boy.