Maybe our universe runs in their VM. I don't know how you can assume that we would be able to detect anyone. If they are smarter than us, they'd know how to hide.
If we run in their VM, then the Fermi paradox still applies--since physics is consistent in this VM and seemingly leads frequently to life, why wouldn't we see signs of it elsewhere in the galaxy?
If the "we're like ants/barnacles to them" theory is true, then they wouldn't bother intentionally hiding from us, because we're no threat to them. If they do it unintentionally, that would still be interesting: for example, it would probably mean that civilizations do not in fact aggressively colonize the galaxy.
It may be electromagnetic leakage or Dyson spheres or any of a number of other signs of artifice.