When there is something defined in .pro file. I mean like this:

DEFINES += _ENABLE_SOMETHING_

And when you're editing some source file, and you have something like this:

#ifdef _ENABLE_SOMETHING_
doSomething();
#endif

then it isn't highlighted (is in GREY color, or editor think that it isn't definied)

The syntax highlighter should check in the .pro files as well for define statements so that defined code will not be displayed as not defined, which could lead to errors and confusion.