Yeah no backlight in plasma but even the best ones will emit some light when displaying a completely black image. You'll have to have the room dark to see it. But that's fully on and running - just displaying a black screen.

I've never heard of plasmas "glowing" when in standby. They generally don't need any kind of warm up phase either so having it on like that is unlikely to be necessary.

If you / they have a power meter, put it on the set and see. Does yanking the plug kill the glow? If so I'd say it's faulty like Tony, albeit a fairly weird kind of fault i.e. the brains aren't turning the display off.

Lastly I don't believe there's any kind of lingering illumination on any plasma that I've seen. Pulling the cable will verify that pretty quickly.

My experience is from a Panasonic and a Samsung plasma.