I seem to remember that if a c_trace returns a negative number it means it is hitting the back face of a polygon. But I cannot find reference to this in the manual...

"< 0 - A polygon was hit from behind. The from position is probably inside a solid object"

Yes, anyone removed from Manual.

I guess the question is whether it was just removed from the manual, or whether it actually works differently now?

I guess, it s just a mistake (found this sentence in german manual)

Yes, I distinctly remember from an older version of the manual...

Yes, confirmed it is in the A7 manual:
"< 0 A polygon was hit from behind. The from position lies within a solid object, or the me entity intersects with a target entity. "