Customer testing
I have a friend who is a manager of another Boscov's store in Pennsylvania. I have indeed seen Sanitaire machines being used in Boscovs stores-- different branches. Boscov's is a department store that has always had a very decent display of vacuum cleaners, similar to Sears. Similar to Sears, they have always had numerous electrical outlets throughout each branch store for customers to play with them as they please.
To be quite honest I think the dirt inside those canisters is quite minimal and looks like demo. I don't believe most department stores vacuum everyday as their stores are so large, I think its only a couple times a week. That being said when the machines are demoed on the regular stores rugs, they will definitely find some dirt. You might also imagine that ordinary janitorial staff is very quick and abusive with machines. They would not be doing a very good job vacuuming regardless what they use and crash, bang into everything. If indeed these machines were used by janitorial staff you would see lots of scuffs and scrapes from hitting different metal clothing racks, etc. With the poor quality vacuuming that many store janitors do, a demo unit will ABSOLUTELY find dirt! It only takes a few demos to get a fair amount in the there. The stores should leave some of that dirt in, as I've seen customers dump the dirt back onto the rug and test several machines to pick it up.
Around 1999 I worked for a restaurant in a mall. I went around to numerous stores in the mall to borrow a vacuum as our restaurant vac broke and the manager was cheap. Sears actually let me borrow one of their displays to clean our restaurant. I loved the vacuum. I borrowed a Hoover Windtunnel S3630 canister. As you can imagine in 1999 most vacuums were all bagged style. Bagless was still too new and only seen in a few machines such as the Fantoms. All of the Sears display units had dirt in the bags. It wasn't from the stores use of cleaning store, it was customers demoing them.
There are more consumers than you think out there smart enough to shop at department stores like Boscovs and Sears where you can actually plug in and use a machine on carpet for testing. Something you could just about never do at a Target, WalMart and many other Big Box store.