[Calgon]

That's not how it works. OnPlayerUpdate just isn't called at all, when a player is alt-tabbed. The only way you can detect if a player is alt-tabbed is to see if OnPlayerUpdate is NOT being called (for example, you could use a timer to get the tick count of the server, and an older version, if the older version isn't somewhat equal to what the check is, then they're alt-tabbed).