by nom_
No, it doesn't stop looking for a focus just because the closest character is surrounded (or whatever), in that case it would just look to see where else it could find a hex to attack someone from, with a path to that hex.The monster will consider all enemy figures, and all hexes it could attack those figures from, that it also has a clear path to. It will then choose whichever wins the tiebreaker conditions (least movement, closest hero, initiative, etc.).
Only if no such hex exists, either because they are all occupied, all paths to any such hex are blocked, all enemies are invisible, or whatever, only then will the monster be unable to find a focus, and not move/attack.