The guest account appeared after a crash restart (with an unusual message and screen that I haven't seen in earlier system crashes). The guest account is now there despite it being turned off in the User settings and despite both Find My Mac and FireVault being disabled. Those would be the typical reasons for a mysterious appeareance of guest account but not in my case. I also haven't had any OS update recently so it's not something official change. So it's unsettling, I'm worried it could be a virus or a hack? It's not the presence of the guest account that troubles me but what made the strange restart with an unusual message and the apperance of the account, I just really want to know if this is a sign that my mac has been compromised?
Update: By Enabling Guest user, logout, then re-disabling it, I've managed to make it go away from the login screen. I'm not adding this as answer because my main question is whether all this could be a sign of my system being compromised and should I take further steps to investigate what caused the crash and the change in my system settings?