On series 3 in a flashback it showed that Orson was at Moniques house when Mike came to do a plumbing job? And when Mike left to get plumbing equipment and came back Orson was there instead of Monique.
IDK it's hard to explain I hope you can remember it from that ^ I'll go into more detail if you don't.
Yeah I think he was just worried about Mike recognising him and perhaps somehow hearing about the dissapearance of Monique and find out she went missing on the night he was there and saw Orson then you know putting two and two together.