Well, for one thing, while the barracks is elongated, the doorway is situated on one of the narrow ends of the building, not the broader length of it, so the view he would receive would encompass pretty much the entire length of the interior. For another... nah, I'm not going to explain that one, actually; let's just say that in this case, seeing clearly despite significant difference in light-levels is not so improbable. It would far from let him see the entire building interior - just the lower floor in itself would be out of his field of view altogether and easily reachable - but coupled with the fact that Aemoten abruptly just came out there and immediately engaged him after an extensive stare-off with Jaelnec and Etakar, it's enough to be a pretty significant hint that something has not gone as planned.
The rune mage has turned slightly more in Aemoten and Etakar's direction (I got the impression they were going the same way? Otherwise it would just be Etakar's direction) than the door.