"Your mother wouldn't happen to be visiting the Bajorans at that table, would she?"

The boy nodded, and glanced nervously at the door, frightened his mother might happen to walk past.

At that time his mother "happened to be traveling by train".

"How did your mother happen to think of that?"

Fortunately his mother was happening by and she retrieved her child.

His mother also just happened to be a writer on the side.

My mother happened to see the woman who'd been my daughter's form teacher for the previous two years, who was known to interfere.

Her mother happens to walk by and quietly watches in the shadows.

The gas inside the house has already killed both of her parents, which the mother happens to be Cornelius's sister.

What's more, her mother happened to be a lawyer.