I'm trying to collect the eighth keyhole in stage "Depression", but it is not there. The list of keyholes in the "Extras/Keyholes" section shows all keyholes filled except for the last one. I have of course not been awarded the "The Truth" achievement.

I've checked several guides and am familiar with where it should be located, but it is not to be found. Restarting the chapter does not fix the problem.