For example, I see "X people chatting" link in this post:
Also, wtf I'm the last poster in the Tavern? And why last message 54 mins ago? Clearly wrong. Last post was from rene and definitely not 54 mins ago.
While there, this link is nowhere to be seen.
I guess that this link appears only on answered posts, but that's unclear why.
Wait, I noticed no link in this recent question. So it also depends on age of the oldest answer to that question? Or just caching?