When do enemies regenerate health?

Question

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)?

Answer 1

All unique monsters/groups of monsters will heal when there are no players within their range.

The only time they do not heal is when engaged in combat.

As for the rate, I am unsure, although it may be slightly higher in the latter difficulties (I can do some testing when I get home from work).