Since these users are on Mac, and the issue is the same in all browsers, the culprit is probably a Mac OS preference. In 10.8 and later the scroll bars are invisible unless the user is actually scrolling by default.
For the affected users, go to System Preferences->General (top row, leftmost item in 10.8). There is a three-option radio button set labelled "Show scroll bars:", with options "Automatically based on mouse or trackpad", "When scrolling" and "Always". My bet is that it is set to "When scrolling".
I had a client using Mac OS 10.8 and FMS12 IWP, and he complained about the scroll bars several times until the message sank in. The irony was that the system was going to be used mostly by Windows users. :-)
Thank you, Rob; that was it!