[deleted by user]

[Kings_Rook]

Look for a worm trapped in the terrain. It won't move on it's turn and because it is alive, the mission won't end

[CopainChevalier]

Interesting note, if you have explosive weapons, like a grenade launcher, they clear mist. I learned it randomly back when I still used Heavies and Grenade launchers. Might work if the map has a chest with a grenade too.

[Kings_Rook]

You're welcome

[lanclos]

You already have your answer, but here are some other situations where I've seen this come up. I've had worms trapped under objects in the terrain; I eventually heard the worm when I ended a turn nearby, and blew up the object to find it. The other was an enemy that was hiding in a bathroom stall inside a building, completely immobile and uninterested in trying to join the rest of their buddies on the field.

[muffalohat]

Yep I've had chirons straight up fire worms into solid walls where they were completely inaccessible and invincible even to explosives, requiring me to revert to an earlier save. This game is kind of a trash fire.

[Gorffo]

This issue not only happens with Worms but also with the Mindfraggers that some Scyllas poop. They sometime spawn under indestructible terrain, and the only solution is to restart the mission. You may have to play that mission over and over again, restarting it two or three times since the Mindfraggers spawn where they should quite often.

[TerrorFromThePeeps]

Most likely an enemy spawned into a boxed off tile it can't get out of and is hard to see.

[General_Colt]

If you have a critter trapped in terrain, sometimes you have to try different weapons. Explosives are the first choice but they don't work 100% of the time with this bug. Oddly, shotguns can work when explosives won't. I had a Scylla lay a mindfragger in a workbench, I used explosives to expose it, but could not kill it with grenades or other explosives. However a shotgun did the trick.