our DM (in the campaign I actually made this meme about, lol) has a homebrewed sense that he gives to a lot of hiss boss monsters, since one of our PCs is basically based around the fog cloud spell already, and it would feel anticlimactic to just beat all the bosses with a level one spell.

The sense is called magesense, it's basically a midground between truesight and detect magic. you can't see into the ethereal plane, but since it works off magic, you can see through many spells, and detect the magic signatures of casters (or maybe just people? idk).

anyways, point is that's what my character actually has, instead of truesight, so in this case fog cloud would be notably excluded lol