A few days ago I noticed Opera icon in my system tray (Windows 10) just after boot.
I had the same on my laptop, but not on my desktop computer.
There is some info about Opera Browser Assistent on the Opera website:
The software may include “Browser Assistant”, a component that provides you with contextual hints about certain Opera browser features and other useful information which you might be interested in. To improve your browsing experience as well as the relevance of hints, this component may collect feature usage statistics as well as feedback on the suggested hints.
I uninstalled Opera with the option to keep my personal data. The Opera Browser Assistent was removed from my computer. Some tmp files and the assistent folder remained which I deleted manually as administrator. Then I installed Opera again. Problem was solved.