Monster with endless Cannot Die aura without a rare mob with them

It happened in a Tora's daily mission. I found some wild pack of normal monster with this mode. I couldn't see any rare mob with them, although I could just have killed him really fast first and just didn't notice it. I couldn't find its corpse, but maybe I had just used flesh offering on him... Not sure about those details... Anyway, the normal monsters still got this mode, and couldn't die. I checked, there was no rare mob near them to keep the aura up.