There is already a system in place.
Online it's a green filled circle with no patterns within.
3 days offline is a yellow filled circle with a clock within.
7 days or longer is a grey filled circle with a black no entry sign within.
I am pretty confident that the guys could come up with something different on their own, without altering the current graphic style.
I can't currently tell them apart easily, I have to hover my mouse to find out what the status is