I talked about this before. I do not beleive this is the only plane where box cutters were found.
I think that there were operatives working for airports or airlines that place them there.
A hijacker did not even have to smuggle them on board -- they were there waiting for him.
Either way, you're right in remembering that boxcutters were found elsewhere, on other 'grounded' flights. I destinctly recall that myself.