Dr Strange!
Stephen came to this place by a more circuitous route. Examining the magic-eating sphere he'd retrieved from SHIELD, he'd had to work out just how he could find magical null spots without using magic to do so. It had been extremely tempting to send out a magical blast that covered the Eastern Seaboard in enough fallout energy so that he could watch for the swirl of magic as the runes drew it in and drained it away - but however worried he was, he hesitated.
Sometimes, it wasn't enough to flood a body with poisonous chemicals and hope that the tumor shank. Sometimes, the answer was surgical, and his form of surgery had never been about hacking open a chest cavity and rooting around - at least not since the bloody old days. He'd always operated with some kind of targetting assistance, and in this case, held the agent in the palm of his hand.
It didn't matter that this particular set of runes hadn't been crafted by the same hands that made the larger ones hiding Loki - probably. He could exploit the fact that they were both the same set of runes, doing the same thing. Sympathetic magic: use a smaller copy of a thing to radiate changes up to the real thing. There were undoubtedly evil ways to use it, but at its simplest, it was about the magical connection that called like to like. He just had to follow the tugged string.
And he wound up about where he probably should have expected; a concrete hallway, a little damp and cold and crushingly oppressive - even without the wards that rippled and seethed like something convulsing in chains. He followed the delicate strands of magic, and when a black-jumpsuited guard with a gun tried to stop him, he flicked his fingers and bound him in the rippling crimson bands of magic, and bashed him against the wall.