Possible Clues:

• One doesn't know them
• Unknown people
• Outsiders
• People one doesn't know
• If there were a further fifty among these unknowns, they would take your breath away
• It makes you cross, half way along the street, to find ones you don't know
• They don't know each other
• A 20 across start to get them roaming the streets, perhaps
• Foreigners
• Subjects of a parental caution