Need help on identifying Firex Smoke Alarm that looks like G6

I was once at my granddad’s friend’s house. I saw a mystery Firex Smoke Alarm (Will be referred to as Firex G-U after this) in every room, with a Firex CB. What does the Firex G-U look like? There is a dim continuous green light on the Firex G-U and I have never heard the Firex G-U go off, All I know is the Firex G-U looks like a Firex G6 with a Continuous dim green light. The Firex G-U system is most likely interconnected, as there are maybe more than 6 of the Firex G-U alarms in my Granddad’s Friend’s house. i found a similar Image of the Firex G-U on the internet, here is the similar image, on the bottom.