This refers to when two or more people encounter each other in completely coincidental fashion. You might notice your old classmate from three countries away is now your waiter in a place you had no reason to expect them in, and you might say “wow, what a small world”. You might notice two people who you know from completely different spheres miraculously know each other. You might recognize by chance that your penpal has made a cameo at a venue you’re at.
But what was your most profoundly coincidental encounter?
It really is. You’ll never hear “someone forgot their umbrella, I hope he/she comes back for it” in real, native speech. Singular “they” has been around in that context for centuries.
Using it for a specific, know person is new. In this case it’s a specific unknown person, so it’s optional, but I chose to, just because it minimises personal information shared. My cousin in not nonbinary, for what it’s worth.