Called person up and he is not in Shul:
If the Gabaiy called up a Kohen, or Levi, and the person is not in Shul or is in the midst of Shemoneh Esrei, then another person is to go up in his place. That person is to go up without his name being called, and whoever comes first merits. Some are accustomed apply the same rule if a Yisrael was called up and is not there. Others however rule that a new Yisrael may be called up by name.
 Michaber 135:11; Ketzos Hashulchan 25:3
 Ketzos Hashulchan 25:3
 M”B 135:38