When visiting Freeside, you'll see a destroyed building in front of Mick & Ralph's, in the other side of this building, seeing one Freeside thug waving his hand to signal you to enter the alleyway. You must follow him, and when approaching him, he runs into a nearby alleyway. If you keep on following him, he will stop next to two dumpsters, and show you a corpse, opening conversation with you, saying "Check it out, between the dumpsters there". You must choose one of the options. Any of the choices will turn him hostile, as well as the three thugs waiting to ambush you at the mouth of the alley. They are relatively weak, and poorly armed. The Terrifying Presence perk can be used to force them to surrender and run away, though they tend to be extremely cowardly otherwise and often retreat during combat even without the perk. If left alone, they may not be hostile afterwards and will act like basic NPCs. Also, if you stealth kill the waving thug before going down the alley, the other two thugs won't become hostile and instead lean against the wall with standard dialogue options.
Edit