I created a screenshot:
Last active poster: You can see an example of the “small grey-ish circle” in the third and last thread of the screenshot.
Seemingly no conversation at all: In the fourth thread it seems as if there were noone to post or to answer, that particular thread though has 2 replies at the time of this writing: How to read error messages?
In the first thread of that screenshot, you can see that my own avatar is slightly off to the right compared to the avatar below. In that thread there also was such a ghost-poster.