Thank you, good feedback. I think I will implement what you said about invert being in a fourth category of techniques. As for what's interactable, I am kind of stumped as to how to walk the like between clear and too obvious. I tried to make it so pickupables glow but then I felt like it took exploration out of it. If you have a suggestion I'd like to hear.
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I recall RPG maker games did those glowing starts but that'd be an overkill. LA Noire did gentle piano note when you were nearby interactable which is more subtle but still might be too much for some. Maybe some suggestion from the suspect, who happens to know that the victim hid something behind the mirror, in another line he could mention number of rooms of his flat to let player know that there might be a hidden one.