Modern AU concept:
*grindr notifcation goes off*
Merlin: Arthur ... Why are you on grindr?
Arthur: Well, you said that I would return when Albion's need is greatest. But I can't do it on my own, so I thought I'll go and recruit some knights. And in the app description it says that you can find men on this app.
Merlin: ...
Merlin: okay then, carry on.
Bonus 1:
Arthur: MERLIN!!!!
Merlin: yes, Arthur?
Arthur: why do all these men want to bed me?!?!?!
Merlin: because it's a dating app. I told you about those
Arthur: oh... Wait, then why did you recognise the notification sound?
Merlin: ...
Arthur: Are you bedding men?
Merlin: I do have a life outside of you.
Arthur: *hurrying to open his phone*
*grindr sound goes off again on Merlin's phone this time.
Merlin: did you just match me?
Arthur: you can't have a life outside of me. I am everywhere.
Bonus 2: *weeks later*
Grindr sound goes off again.
Arthur: holy shit, I think I found Gwaine!!!