The Lost Boys is one of the early side missions you'll come across after you visit Zalissya in Stalker 2. During the Stalker Wisdom side mission, you'll speak with Lens, the technician, and be able to undertake another side mission where he'll ask you to find some diggers and an NPC named Mastiff that have gone off radar...
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Stalker 2 The Lost Boys: Should you help or kill The Ninth?
Source: PC Gamer (opens in a new tab)