DH dreamed the other night that his ex from many (20 ish) years ago had a daughter that was his and he's never been told. He said it could actually be true, as they split quite suddenly (both had drink and drugs issues) with no further contact and she moved back to another country.
It made for an interesting conversation and we talked theoretically about how we would handle it. Obviously it would depend on what theoretical daughter wanted!
I told him to look his ex up on social media to put his mind at rest (DH tends towards paranoia!). Well, he found her profile and there's a photo of her and a girl who could be anywhere from 15-25 years old. Profile quite secure so can't see much else. I said if you're still worried then message her and tell her about your dream, she'll either laugh or tell you it's true...