It's an automatic reaction, the subtle increase of heat around his body, even while he clamps down on his IR radiation. With Lucifer, he didn't block the radiation but that had been something which had developed as he had realized Lucifer enjoyed the heat. Still, it was reflex to be as invisible as possible.

While Hitsugaya worked, Magneto watched while adding an obscene amount of salt to the second batch of holy water. It was going to be more salty than the Dead Sea, which he wanted. Once the first batch was finished, he carefully poured it into the first bucket, letting some play though his fingers. With his infra-red radiation blocked, he didn't need to worry about it melting. "Wonderful. This is perfect."