Randumb idea for you folks.

Half elves are a misnomer. Most unions between elves and humans result in a human child, but the elven trace remains in that bloodline for many generations. An elven child can be born at any point from this bloodline. A fully elven child. A great great great great great gre- grandparent might show up on your doorstep one day when you have an odd, pointy eared, child. You'll be expected to give it up.

Though with elves ever prone to crisis, you may be raising an elven child among humans. Finding the lost is really a factor of if someone managed to scry for them.