Anyone would think we'd asked for a bottle of champagne, when all we wanted was a packet of coffee, which was inaccessible because it was displayed behind glass under lock and key. A shop assistant had to open it up to free a packet for us. The price of raw materials has, of course, soared in recent years, but inflation doesn't explain everything. In some American supermarkets and drug stores, items such as shampoo, men's socks and detergent can only be bought by asking a sales assistant to open the display case for you.

Self-service stores, which were an American innovation, are now seeing the pendulum swing back, with aisles entirely under lock and key. There are no sales assistants, but you can usually press a button next to the window to summon someone. And yes, I've seen an employee unlock a display shelf for a woman to grab a $2.99 exfoliating shower sponge.

A sign on the display window in the men's socks department of a Target store in Manhattan, doesn't say: "To prevent theft, we've locked everything up." Instead, they have: "To maintain our inventory levels, this section requires the assistance of a team member," or "Secure shelves keep products in stock so you can shop." The entire toothpaste section is behind glass. A man in front of me asked: "Do people even steal toothpaste these days?"

It's not always what's most expensive that is locked away, but what's easiest to sell on, which is what the store Giant in Washington DC said to justify its decision not to sell certain brands, such as Colgate toothpaste and Tide laundry detergent. Clearly, private-label brands are less easy to resell.

Items such as vitamins, Red Bull and ice cream are often under lock and key. In Manhattan's Fairway stores, Häagen-Dazs ice cream is sold with an anti-theft device on the lid. And so, the world now has engineers whose research and development departments require them to work on anti-theft devices designed to prevent a 400 g bucket of ice cream from being opened.

In the same stores, a large padlock has been put on the dietary supplement shelves. Never have chain stores been so aptly named. Perhaps all the padlocks could have boosted shopping – after all, they could make you feel like you're Place Vendôme, surrounded by all those luxury products displayed in the windows: Shampoo! Deodorant! Paracetamol! The truth is you feel more like a would-be thief. This ruins all the efforts made by supermarkets to make us believe that cashiers are team players, that sales assistants are advisers, and that shopping is "an experience."

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