I've noticed this has been happening since 2.0 too. Only happened once during an actual mission though, thankfully, that was the Bozob mission. We cleared all the enemies but Bozob wouldn't move, so I checked around and could hear the enemy beneath the world. I ended up triggering the nearby police and got into an epic 5* fight where MaxTac turned up. Oof, they're all bosses. That didn't last long lol.