It actually takes a while for him to figure it out and even then he only has his intuition to go off, (unless the Kwami do their seance thing to communicate with Tikki).
All Fu knows is he gave the Miraculous to Marinette and within the hour there’s a high on herself blonde obviously NOT MARINETTE running around in spots, WHAT?!
When he figures out that is was theft….
“I’m not taking the fall for this one Wayzz.”
“But Master, would it not have been better to put it in her room and not her bag–”
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