This looks to be where the functionality for that icon comes into play - basically just telling you they aren't nearby. So either phased, in another zone being summoned or recently refused or accepted a summon.
https://github.com/Gethe/wow-ui-sour...Frame.lua#L563
And this is it's place in the xml layout for the partymemberframe.
https://github.com/Gethe/wow-ui-sour...lates.xml#L426
Looking at grid2's patch notes ( says phasing stuff added to classic version ) and the retail version code (searched for phas and notpresent ) but nothing in the addon mentions those. So it might be that what you have seen in grid2 is something different.
How to add it to grid2's frame itself , I would suggest seeing if someone updating the addon ( last update was this month ) would look into adding the feature ( if not already ) .
It might be something as simple as reparenting but seeing as this is an in combat related addon, I'm not sure if there are other things that will need to be considered as well because of that.