Just to wrap this up...
Customer service responded to my question. It seems that no unticketed passengers got on this flight. Apparently, the two guys I heard were receiving upgrades...they had already been ticketed.
Here's a surprise, though. Quoting NW's customer service:
"Additionally, for future reference if you are in this situation again, the stand by list for an alternate flight is called strictly by the time a passenger is added to the list."