Hi I am new on forum so my language will be very simple...

I am opensuse Tumbleweed user - Yesterday plasmashell process started to eat a lot of CPU and memory so I killed the process and started from console being on foreground of one console tab, at some point it restarted without my intervention (taskbar disappeared for 1 second and reappeared) and when I stopped process plasmashell was still running. Is it normal or someone hijacked my kde session and is looking on my screen without my knowledge? I don't have any idea how to check it, maybe it's Xserver related?

While I wouldn't call such behavior normal, I wouldn't expect someone hijacked our kde session either (if some hacker got into your computer, hijacking your desktop session to monitor your habits would probably be on the lower end of their priorities as there's a lot more to be had by e.g. copying everything in your home directory over to their servers to look for passwords or other sensitive information).

In the console where you launched plasmashell, did that process actually exit? What output has it produced?