I have a system always blinking too, About 10 o'clock if the milky way were a clock, near the outside of the sector with the Citadel. called Minos Wasteland. For me it's currently at 100%, no missions. I went there anyway and found two systems, one called Fortis, the other Arrae, and both are 100% explored with no active missions in either. A glitch maybe? That seems to be the only one for me currently outside my only other active mission.
To add one more thing, if you pick up on conversations around the Citadel people ask you to find crap for them (or you eavesdrop about it...) Sometimes systems with the items they require will blink too. These won't be 100% explored, but I'm not sure if you can find the item before gaining the quest...