I've noticed while waiting to respawn that some enemies regenerate health when no one's around (most without the Vampiric Modifier). I believe I've seen all enemies classes do this at some point (Champions, Uniques, normal, etc.), but I've also seen enemies not regenerate health when I'm dead.
Is this a Boss Modifier they have in common, or is it unique to certain areas or party conditions (e.g., everyone dead)? I feel like I've seen this more often with Fire Chains enemies (or something that looks like Fire Chains), but the recollection is vague. Do all enemies regenerate their health upon leaving the current zone? Can shamans and enemy resurrectors replenish life? Can Champions+ regen life for normal enemies? And what affects these rates of regeneration (e.g., difficulty, proximity to neighboring enemies)?