Mod show automatically send a chat command that someone is AFK once after a configurable amount of time. By default 5 minutes. Something like "[nickname] is afk since more than 5 minutes". After that chat message, no more messages should be sent automatically for that player until the player gets back to game.

I'll make it at some point.

Probably, I'll make it after I'll make a mod to show players via cameras.

Actually, probably, I'll make a mod with a similar functionality.

Perhaps, you'be interested in this https://mods.factorio.com/mod/Players_info
"Play time" is gray for AFK players.