A few weeks ago, members of the cloud computing community flooded into Las Vegas for re:Invent, one of Amazon's largest Amazon Web Services (AWS) conferences, where the company shows off new products and throws parties. But attendees weren't just following what was happening at the conference—the conference was, in a sense, following them.

Included in re:Invent's attendee badges was a small device acting as a Bluetooth beacon, which allowed AWS to track people as they moved from room to room. AWS said the beacon was anonymous and was used to help understand foot traffic and attendance at certain events.

Bluetooth tracking or crowd analytics is certainly not new, and at least some signs around the conference mentioned the tracking. But some attendees said they weren't aware of the beacons, and would have opted-out if they had known about them at the time.

"Had I seen AWS themselves describe it as a 'beacon', I'd've known exactly what that means and opted the hell on out of that one," one attendee told Motherboard. Motherboard granted the source anonymity so they didn't face professional issues for speaking about the topic. The attendee of the conference provided Motherboard with photos of the badge and the device it contained.

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"We knew something was different this year because they were very clear that if you lost your lanyard, you'd pay $150 to replace and it would be hassle," the attendee said.

"WTF," one message in a Slack chat of attendees, and shared with Motherboard, reads. "I feel violated… LOL," they added.

The source said they researched the beacon, and found it was acting as a service called "TurnoutNow."

AWS confirmed to Motherboard this was the company behind the beacons, and added that attendees could opt-out if they wanted to. AWS said the beacon is not associated with any personal information about a particular attendee, and only provided data to an AWS team focused on events operations.

TurnoutNow clients can "View event-wide behavior trends or get granular to a booth or session," and provides "Exhibitors with booth performance and ROI [return on investment] analysis," according to the company's website. TurnoutNow did not respond to a request for comment.