Referee Rob Styles revealed that the Leeds striker was at the centre of a tunnel bust-up with Patrick Vieira.
Smith had angered Arsenal players with two rash challenges in two minutes and they confronted him at half-time in the FA Cup third-round tie.
Styles said: "I saw Patrick Vieira and Alan Smith talking to each other. It was a very short exchange of views in the mouth of the tunnel and going a few yards into it. I was on the scene quite quickly and walked with the lads up the tunnel. I saw what I needed to see and I managed it at the time.
"I saw everything and, as far as I'm concerned, there is nothing to report."
Smith as given his ninth yellow card of the season on Sunday.