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I get the 4 hostages out, and can't complete the mission. Normal mode. There's a white dot where a 5th hostage should be but he's not there.
Checked Rainbow Six wiki and apparently the little boy can separate from the others but I searched EVERYWHERE on the map with no sign of him.
Watched Youtube vids and in the comments I have seen others have this problem and they say it may be a glitch.
That mission is bugged, unfortunately. One of the hostages and a couple of the terrorists will not spawn so you can't complete the mission. Your only option is to use cheat codes to skip it.