The great assumption is that advanced civilizations will remain in material form. It could be that very advanced minds necessarily migrate to some kind of virtual environment and continue the quest for the Final Truth [which seems to be what intelligent beings do] as tenous pulses of energy. The only material need would be the 'server' in a remote corner of a run-of-the-mill planet that would host the virtual mind-beings. They wouldn´t be noticed as a material presence in anywhere else. That would solve the Fermi paradox and do away with the 'Great Filter'. This concept has been already explored in many SF stories, and I don´t see any reason why it couldn´t be.

Edit: Bolstrom, as a 'transhumanist', should have thought about that, but the idea is absent from his essay.

__________________
What brings us together is stronger than what pulls us apart