Even so, he would still make noise. Yeah, he didn't have to walk all that way so they wouldn't have as long to hear him, but still - armor's loud and how don't you hear the armor? Remember this strip? Those goblins heard him all the way away.
He probably just cast the spell on the same round as he popped out of the wall, since it just takes a standard action. Even if they could hear the armor, they don't get to interrupt his round of actions till after he's finished.
"Mad Science" means never stopping to ask "What's the worst thing that could happen?"
- Maxim 14 of the Seventy Maxims of Maximally Effective Mercenaries. From Schlock Mercenary by Howard Tayler.