It could be something else, but the timing is hard to make work because Chad's immediate response is to go after the MC in a blind rage.
To that point, the only things the MC had done that would get him on Chad's radar are 'hitting' on Sage in the cafeteria, and being caught spying on him in the locker room. If you assume he learned about the sparring session with Sage, I guess you could add that too.
The problem with the Sage connection is his tear filled line after the fight. If he thinks the MC is stealing his girl, surely it's obvious what the MC would want from him, no?
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It sure seems like eavesdropping on Chad's phone sex is the only thing that could explain both Chad's rage and his fear. Is there anything else the MC is conspicuously enough involved in that Chad would associate it with him, yet also holds some dark secret Chad is terrified of revealing?