I can confirm is was the pidfile for me. (For others: It's located in the root of your nodebb directory.)
I had restored from a backup and the pidfile was copied into the new directory.
If the topic is long, and someone is browsing, the topic is too close to browsing and the browsing user avatar. Can you imagine if three of four (or more) were browsing?
See "watch" and "browsing:"
Also I noticed the "browsing" avatars don't display on mobile or tablet. And that's probably due to the lack of space. There are a couple of options: use the Bootstrap grid markup or an alternative is flexbox