Think it could be a leakage, but how serious?

Filled the car up fully a week ago, and Im already down to less than half a tank, I haven't made any extra journeys so a bit worried, also there's no puddles around the car or if its been sat, theres nothing when I leave. It did this last time I filled it up to the top, the first half disappeared fast, then the second half slowed down and lasted another 2-3 weeks as I only use the car to go to work and back.