As Lowell states, they're not really orphaned.

Extended properties that aren't explicitly subordinated to a db object apply to the db itself.

That is, if you add extended properties w/o specifying a specific object, they become extended properties of the db itself, so only dropping the db would drop those extended properties.

Otoh, if you add extended properties to a column, for example, they "belong" to that column and automatically get dropped with the column (or owning table) itself.