At the motel, Flat had claimed that he could see something like a mist of mana covering the whole city. Even when he transformed into a mage, however, Berserker had not been able to sense anything out of the ordinary. It seemed that Flat, however, could see those alien currents.
“This isn’t ordinary Mana,” he had declared with uncharacteristic seriousness. “How can I describe it...? It’s like if every drop of drizzle was an independent living thing... Or like if the town was covered in a swarm of super tiny locusts...
“At the level it’s at right now, instruments for measuring magical energy won’t register anything. If the ‘fog’ gets about two degrees thicker, though, I think mages with sharp senses will start to notice it. Even right now, a very perceptive Heroic Spirit or something that perceives things differently from humans — like a hematophage — could probably sense it.”

Fate/Strange Fake: Volume 3, Chapter 9 ("Day 1, Evening")