I knew a girl growing up who met someone older on Bebo (a relic of another time...) and ran away with him before being caught by police. There's also the possibility of the Vampire Freaks social media site back then, which was shuttered due to its connections to abductions in the US, I believe but
was also used in the UK. I used it at a similar age to Andrew and had unpleasant older men try to interact with me, so I closed my account.
Personally being in the alternative, goth and metal scenes I can say that whilst 99% of people going to gigs are wonderful caring people who would report/defend a young teen alone, as with any group there are a 1% that would jump at the chance at taking advantage of a young vulnerable person on their own in an unfamiliar place, offer them a place to stay (with nefarious intentions) and/or let them 'sneak in' to a gig and claim to be an older sibling having met outside a music venue. Particularly if you are going to the box office to purchase a ticket in person, which would have been a necessity for Andrew, that would provide an opening for sketchy characters out there who seem harmless on the surface and even helpful, but turn out to be predetors.
For example: "oh hey you're going to see Sikth, great taste man, me too! Do you want to go together?" etc.