That sounds really sus. You mentioned why they would have asked how long you've been living in the house for, but I still feel like that's such a strange question for a landlord. I'm a landlord as well and that just rings alarm bells for me. Unless there is an actual issue, there's no reason for them to ask that since they should know, at least roughly OR could have confirmed it with the agent. And there is no reason whatsoever for them, as the owners, to go into the backyard without knocking first. Your poor step kids must have been terrified.