(UPDATED) Another day, another Apple rumor. This time rumors are swirling about Apple’s clandestine activities at its retail stores. According to several posts, including ones on Fast Company and Apple Insider, both stemming from an original post on Boy Genius Report, the Apple stores are gearing up for something big the weekend of May 20-22. The prediction is that Apple's proprietary point-of-sale system will be receiving an upgrade, one that might allow it to accept contactless payments via near field communications (NFC).

Citing a "solid Apple source," Boy Genius posted the following Apple activities on its site on May 17:

• There’s an overnight shift planned for around 10-15 individuals at each Apple Store to work from late Saturday all the way through mid-Sunday.
• During the overnight shift, it's going to be required that employees lock cell phones in the main office. They will also have to sign an NDA with Apple.
• There are a wide variety of roles, we’re told, for the overnight shift. This includes all visuals staff, a manager, a business team member, a few Genius team members, one back-of-house employee, and a few generic Apple specialists.
• Apple stores have apparently already received hardware to install, and are expecting more hardware to come on Friday or Saturday.
• All materials that Apple stores have received have been instructed to be under lock and key until after close on Saturday night.
• Apple employees will be putting up black curtains at all stores so that people walking outside cannot see inside.
• Employees have had to download gigabytes of data from Apple corporate labeled, "training" in a password-protected zipped folder that won’t be accessible to managers or anyone else until Saturday afternoon.
• Lastly, all Apple retail stores have mandatory meetings on Sunday, May 22nd. Most meetings are scheduled for the morning, but there are evening meetings as well.

Online trafficking of Apple rumors is a virtual cottage industry, but this round of buzz seems to have taken most Apple watchers by surprise. No new product or upgrade to Apple's line was expected at this time. PCWorld has put together a list of some of the product launches that have been discussed in the past that might be rolled out on Sunday. The list includes a new MacBook, a special-edition iPod Classic or even the launch of Apple’s iCloud Web services. However, given that the secrecy is focused on the retail stores (not where Apple typically rolls out new products), and that the big day seems to be a Sunday (not a busy day of the week for product launches), it's unlikely that Apple will be rolling out anything product-related, lending credibility to the idea that the stores' POS systems are a target of the activity.

If the Boy Genius source is correct, and Apple's POS systems are being upgraded, it's no guarantee that the changes will involve NFC. Recent "will they/won't they" speculation about Apple including NFC capabilities on the new iPhone 5, scheduled to be released in the fall, may be fueling the rumors. It is as likely that changes to the POS system may simply mean sales associates are to be equipped with special iPads for the retail environment to replace the specially-equipped iPod Touch units currently used in the stores.

All of this secrecy coincides with the 10th anniversary of the launch of Apple's retail stores. The first two stores were opened on May 19, 2001, in McLean, Va., and Glendale, Calif. Considered a bit of a gamble when they were first opened, Apple now has more than 300 retail locations around the world. The stores account for around 13 percent of the company's revenues, or close to $3.2 billion.

Update: It looks like the prediction of an iPad POS system is at least part of the reason for the activity. Apple Insider reports that retail store associates will be using special iPads with Apple's proprietary RetailMe software possibly phasing out the current iPod Touch system.

(Here's a slideshow of the big secret: iPads integrated into the store's customer management system to help shoppers browse and even request help from an associate. It's not really about mobile payments anymore, but it's still cool.)