There are no hidden users for internal engineers. There are following options:
Activation of access on a development device: A root access is created by specifying the serial number. The device is now permanently stored in our database as a development device. This mechanism is intended for our developers, but not for putting a device into operation.
Temporary activation in case of service: In exceptional cases, it may be necessary to create a temporary superuser in case of service (for example to modify system files). For this special exceptional case, there are "service" assertions that allow a time-limited root access (usually 1-2 days initially) which is automatically deleted.
EDIT: See this thread for news to this topic.
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