You can put a lot of things in the condition of presence line. For example, I often have something like
alive grpLeader1
where grpLeader1 is the group leader of the unit in question. The loon will appear only if the group leader is present. This is handy for making random patrols: the group leader is on probability of presence, so he may or may not appear. If he does, his whole patrol appears, but if he does not the whole patrol is not present.