In fairness and I don’t know what OP does or doesn’t do in terms of checking phones etc, but teens can be really sneaky with hiding things.
Snapchat as an example, messages disappear, you can change someone’s name so mum could check and just see “Carly” and no messages when actually that is 22 year old Dave. Even Whatsapp has the disappearing messages feature. There are forums/chat rooms (not sure what you would call them) but where people male & female post their usernames for chatting/meeting up, even parental controls on wifi wouldn’t always restrict these because they are not inherently adult sites.
No phones in bedrooms is a good solid rule though and is what I’ll be doing when mine are old enough to have phones. From 7pm phones are downstairs and locked away until morning etc.
And there’s no way of knowing 22 year old knows he is talking to a 13 year old, she may have pretended to be older in messages/photos especially if she looks older. If nothing else in the current days with the hunter groups on fb etc I would hope not many people would happily chat to and agree to meet children.
A good idea to know where children are but honestly I don’t know many teens who always knew exactly where they were and who with truthfully, I know I told my mum I was with a friend at one place when actually I was at another. I suppose now with find my friend available it would be harder to lie but if she disables that on her phone…
I think it is hard to win honestly. I feel sorry for my mum when we were teens now thinking back and I know it will be hard when mine are that age. You push too hard and they rebel, you don’t push enough and they go too far, it’s a fine line.