During regular operation, an observer sees a 16-bit short address that is deliberately not unique to one device, it is only a hint the network uses to narrow down which key to try, not a reliable identifier from outside. The one exception is the attachment request itself, which is sent unencrypted and does contain the device’s full, globally unique identifier, though only once per attachment, not continuously. Payload content stays encrypted at the network layer always, and additionally at the application layer if that is enabled. As with any radio system, an observer can tell that a transmission happened and roughly when.