This is curious. The obvious answer is that Timmy has magic, but Noah would surely be able to sense that unless Timmy has some incredible masking ability.

If he's a non-magical nightmare, that still doesn't explain how he broke the grate silently. And Ciel is strongly suggesting that it had to be magic.

@Abbie There's always the possibility of the grate itself being the nightmare, as Noah can only detect them upon appearance, if it dissapears/moves he would still be non the wiser

Maybe the grate was already damaged, but the hole was covered with an illusion?

Rusty: "Eat Grate?"

We know it's not the Malhart illusion girl since Noah would've detected it, too.

Maybe Timmy is someone who's naturally super strong? And Stealthy?
There's magic and espers, but there's also Ghouls and Burk in this world.

A nightmare with some kind of noise-cancelling ability seems possible

Timmy: the nightmare of taking a chance at socializing with someone and they turn out to be mass murderers in disguise

I'm guessing that the grate is actually a nightmares mouth. It was closed when they walked by but now its open and they are being eaten.