A flight plan doesn't have much to do with permission to fly somewhere. It's more about ATC not being surprised if an aircraft shows up in a given airspace, and

A flight plan doesn't have much to do with permission to fly somewhere. It's more about ATC not being surprised if an aircraft shows up in a given airspace, and search & rescue noticing if an aircraft doesn't show up at its planned destination. All aircraft have separation and avoidance rules that apply at all times.