It’s very much observable - you’re observing it right now. Consciousness is an undeniable fact of being human, but it can only be observed from the perspective of the conscious being itself. There’s no way for you to prove anyone else is conscious because it can’t be measured. They can seem to act in ways that suggest consciousness and they may claim to be conscious but whether it actually feels like anything to be them is something only they know.

Medically speaking:

Unconsciousness is a state in which a living individual exhibits a complete, or near-complete, inability to maintain an awareness of self and environment or to respond to any human or environmental stimulus.

Source

There it doesn’t imply the absence of experience. You simply lose awareness of yourself and your environment, but you may still keep having dream-like experiences in your mind.