Yes. It will report 100% when unplugged.
In my other variant, I did have something similar to what you envision using the IAS cluster. Zigbee has some cluster that you can adopt to detect switch between DC or main power. I do not remember whether Power Source information is "report-able" in Power cluster. IAS DC/Battery cluster is report-able attribute.
Are you looking to find out when the sensor is powered by DC or Battery? If yes, there is a couple option using either the Analog Input. There is a pad (let me check with the schematic and get back to you) that will output 5V when powered by DC. The same pad with go to 4.2V or less when it is powered by battery. By detecting the voltage, you can detect whether you are on DC or Battery. I believe I also have DTH on my my github for this.
The 4 sensors (light, temperature, humidity and pressure) will auto update. Diagnostic Attribute such as packet loss and etc has to be manually refreshed. This is based on Zigbee Diagnostic cluster.
Thanks
Iman