Could a civilization hide inside an asteroid?

Yes, in principle a civilization could hide inside an asteroid — but whether it would be easy to detect depends on how active it is and how far away it is.

A hollowed-out asteroid could provide shielding from radiation, micrometeoroids, and outside observation. If the inhabitants lived in enclosed habitats inside it, they could potentially keep most of their structure hidden from direct view.

The main limiting factor is waste heat. Any civilization that uses energy must dump heat somehow, usually by radiating infrared light into space. That makes a busy, energy-intensive civilization hard to hide completely, especially if observers are close and have sensitive instruments.

So:

• Physically possible: yes.
• Completely invisible: not if it is active and being looked for carefully.
• Could it hide from current human technology if very far away, such as in the outer Oort cloud? Possibly, yes — distance and low activity could make detection much harder.

So the short answer is: yes, a civilization could plausibly live inside an asteroid, and it might evade detection if it is quiet enough and far enough away, but a highly active one would still tend to give itself away through heat.