Trip, I've had it before. Exactly this.
I flagged. Referee came over and asked what had happened. I'd explained that the player (despite not touching the ball as it sailed past him), was standing in an offside position. I couldn't gauge exactly his position in relation to the goalkeepers, so when the referee blew his whistle to acknowledge my flag, I signalled him over.
I said exactly what I saw "Player is offside, however I can't tell if he interfered with the goalkeeper. The referee replied "Great call. From my angle, he was between the keeper and the ball".
I'm sure the assistant and Mike had a little chat in the comms but had deemed it okay. Personally, I believe this was a much easier call to make than the subject offside, but one which weren't made through reasons beyond my knowledge. That's the beauty of being humans. Video replays - any doubt, this goal would not have stood.