Scala does not require the developer to use override on methods that implement abstract members. Nonetheless, it is a good idea to mark such methods with override to be notified by the compiler when the override relationship gets broken (by a subsequent refactoring, for example).
This inspection reports functions that override abstract members but do not have an override modifier.

Before:

trait Base {
  def fun(): Int
}

class Impl extends Base {
  def fun(): Int = 42
}

After:
trait Base {
  def fun(): Int
}

class Impl extends Base {
  override def fun(): Int = 42
}