The only time in season 1 when Peter is anywhere near D.L. in order to get his ability is at Kirby Plaza. He was a little bit busy with not exploding, stopping Sylar, etc. in order to notice who all was near him and what powers he could have, or couldnt have absorbed. Plus, he never saw D.L. use his power either. So, how could he possibly know when he was locked up at the company that he could phase through the walls to escape? It makes no sense.

I have this theory that he accidentally phased through his bed to the cell floor on day three without the meds.

He didn't know. Look at his face when it happens- it was a complete shock.

yaaa means when he does it for the first time chronologically - when he leaves his Company cell with Adam. There, it was definitely part of the plan.

That's the time I was talking about. He was completely shocked that he phased into the wall.

You think so?

But how do you explain Adam's "It's safe enough to try"? What do you think they were going to try? The whole scene looked to me as if their escape was all built upon Peter being able to walk through walls.

If anything, I put Peter's expression down to discomfort at passing through a wall (we know from Niki/Jessica that it feels pretty weird when you aren't used to it), and/or concentration, but he didn't look like Peter looks when he discovers a power he didn't know he had.