A friend returned from San Diego Comic Con 2018 with an RFID bracelet used to track users in the Amazon Fire TV experience (on Twitter, #FireTVSDCC). This is a teardown of the bracelet after the event. At this time, I was unable to read from the bracelet.
The bracelet is fairly simple with a cloth band and plastic/paper tab threaded through. The closure is plastic and one-way. It bites into and mangles the cloth band if you attempt to remove, but you could probably shim it with tools and practice. Might be a fun thing for the Tamper Evident Village if it turned out events were trying to use this for access control like plastic self-destructing wristbands.
The back contains a serial number. I would like to see if this serial number would match the data read off the tag.
Separating the badge by prying them apart, I spot the prize: an adhesive RFID tag placed between the glossy plastic covers. It appears to have a model number of "CXJ-040" in the center of the tag. It uses a circular antenna. CXJ is the initials of Shenzen manufacturer Chuangxinjia. Their product pages show many similar wristbands in a few different frequencies.
The tag didn't respond to my Android phone, so it is not a Mifare or similar. Hopefully I can find a reader at the local Hackerspace or DEF CON 26.