I always wonder that. I think some people close their profiles before they crb. Others maybe have a confidante that relays the info to mods. Pure guess on that front though.
Some people do close their profiles, correct. And I recall seeing other people tell mods that someone has ctb'd but I'm not sure if the mods will close or delete the account in that case.
I've seen some people write things like "I'm gonna ctb today. Mods, if you don't see any activity from me during the next two weeks, consider me dead and close my account."
But I don't really know how things work if someone just ctbs without telling anyone about it. Iirc I was once inactive for a long time and no one closed my account.