The thing is, thankfully, that Darien IS good at one thing, and was even before he got the ability to make himself invisible: he's a damn good sneak. In fact, as long as there isn't a moral quandry around to trip him up and make him do something that will ultimately screw him over, he's an exceptional sneak. Part of that is a talent for remaining unseen regardless, and the other part is a damn good sense of when people are getting close.

That's why he kicks the gland in gear and gets himself quicksilvered up as soon as possible. It's also why he reaches a hand over and uses the extreme cold of his current form to produce a small patch of ice just where the creeping footsteps are coming from...