Any Slack users -- a bit of help?

The organization I do volunteer work for now uses Slack for group messaging and to coordinate events. I have it on my Android phone and on my Windows 10 PC using Opera browser. We had no training in it, mostly it seemed fairly straightforward.

On the PC sometimes the logo in the shortcut will be modified. Sometimes there is a solid red dot in the upper right corner, and sometimes there is a hollow red dot in the upper right corner. I can never find anything new on Slack when I get these, and they persist sometimes for a day or two. When there is something new on Slack it doesn’t seem to do this. I get notifications on my phone when there is something new, and if I’m at home I prefer to look it up on the PC. When that happens, there is no modification of the logo on my PC.

The people I deal with at the organization don’t know any more than I do about it. I’m hoping folks here can tell me what, if anything, these dots mean.

I don’t have a good answer for you. For what it’s worth, I use Slack at work on two computers and my phone. I don’t allow my phone to notify me of new/unread messages so I can’t speak to that behavior.
Notifications of unread messages seem to persist until cleared on each computer, at least. I use it in Chrome, and the icon in the tab changes to a red dot or exclamation mark when things happen. Both computers will show this. I read the message on whatever system is convenient. When I go to a computer that I haven’t read them, it still shows the dot or mark in the tab. I click on the tab, it refreshes the status and removes the notification.

Because of how my day gets structures, I almost always have an unread notification on my work computer when I first get to my at-home desk, but there are almost never actual messages. It’s just the way the system seems to be set up.