I have to question whether
Pippin had anything to do with it. When the Company enters
Moria there is a "shattering echo" when
the Watcher closes the doors, and they weren't being particularly quiet as they walked through the
halls. I think it at least plausible that the
orcs were aware of them before Pippin's rock. I also have a theory that the Fellowship heard the
tom-tap,
tap-tom because of the well. There might very well have been signals before and after that, unheard by the Company through the thick stone
walls.