Snape still has the Disillusionment and silencing charm surrounding himself as he steps into the kitchen, so he's relatively hard to spot if it weren't for the dust and debris in the air and the newly created door in the wall. If he had the time, he'd re-transfigure it back into a wall and cast an illusion of a still open door over it, but there's no time for elaborate games right now. Instead, he leaves it open as a sort of psychological test to see what kind of man he is dealing with.

He moves lightly and quietly through the kitchen, eyes drifting towards the kettle on the stove before he sweeps his wand through the air with a murmured, "Homenum revelio."

Snape may be hard to see, but the feeling that some large bird of prey is swooping low over Peter and immersing him in its shadow certainly confirms his presence.