If you mean what's in the attached picture, then yes, it means there's a Mirage Spot or Legendary Portal :) I'm pretty sure the one you're referring to has the 3 Johto Dogs. They're sometime random, but sometimes you need to fulfill requirements to get them to appear. For example, for that one, you needed Lugia or Ho-Oh in your party.
Source: I play that game too often.
Hope this helps!