Re-entering the field of play without the referee's permission.

A player re-enters of play without the referee's permission. The player then interferes with play without touching the ball. What should the referee do?

Assuming the player needs permission to re enter, stop play unless an advantage can be played. (In case of advantage I would say it should be a very clear advantage and definitely at or near opponents PA).
If play stopped YC for entering the field of play without permission and restart with a DFK (to the opponents) where the interference occurred. (Law3)
If advantage played, YC at next natural stoppage and restart according to whatever the stoppage was for.
Circs are a little vague, but I think this covers it, though as always the opinions of learned members would be appreciated.

(I saw a similar incident a few years ago - it kicked off big style when the DFK resulted in a goal being scored directly from the restart).

Just to add, if the interference DOGSO then the sanction would be a red card and if in PA then PK.
And if the interference is a reckless foul, or a careless SPA then it's a yellow for re entering and then a 2nd yellow for the foul offence committed

I think the answers you've received are very much sufficient.