Just to put this to bed, I've figured out what's happening.
I don't have Solaar installed, but I do have the source code. Once in a while I have some time to mess around with OSS development and I fire up Visual Studio and do some debugging in Solaar (trying to figure out what's wrong with it and Budgie, as it doesn't work properly on my system). Well, that's what creates the icon. I guess running Solaar causes the upower callbacks on Budgie and makes it show the icon.
So, on a fresh boot (not so often) the icon disappears. When some days later I get time to play with Solaar code, I run it from within my IDE and the icon appears. At some point I notice the icon (usually much later) and I'm like "well, how did this guy come up?"
Mystery solved.
@eye4bear I have a Manjaro installation on another partition that I boot into to check distro issues when a VM will not suffice and indeed Solaar works fine there for me; so it's definitely related to Budgie I just haven't figured out why yet. I'll start another thread once I have concrete info, but can you maybe elaborate on the issue you had?Mine is that the X input system seems to go haywire the window manager doesn't direct mouse events (I click on stuff and it gets ignored or goes to another window, or is disregarded altogether). It's hard to describe, I was thinking of making an OBS video to show it, but right now I'm trying to figure out at what point in Solaar code this starts happening.