The woman looked like any other grocery shopper that day until the general manager of the store, after a tip, peered at the feed from the security cameras.

The manager, Joe Holtz, described what he believed he saw: “A person walks around and puts some things in a basket that they were going to pay for, and other things in a bag that they weren’t going to pay for.”

The scene was the Park Slope Food Co-op, an institution that exemplifies the utopian, we-can-do-it-ourselves ethos of 1970s Brooklyn as few others do. The Co-op is open only to its members, who number 16,390 this week — close to the peak — and must work 2 hours and 45 minutes every four weeks in an oft-written-about arrangement that dates back 40 years. This practically eliminates payrolls and keeps prices lower than at chain supermarkets. The quality tends to be high.

“We were just young people who were aware of better food,” Mr. Holtz recalled of the heady early days.