The fuel lost per mile would only be maybe .05 gallons, or .2 liters per mile. Seems like a small drip somewhere that could be exacerbated by increased flow when cruising might be a likely culprit. Has the fuel tank been dropped recently? I'm wondering if it's leaking at the lines coming from the pump, like maybe a bad O ring or something. I think it would be possible to not notice a leak back there if it's not dripping to the ground, though I'd think you'd be able to smell it. I suppose it depends on where it could be leaking - it might be leaking on something and evaporating/burning off before it hits the ground or creates an amount large enough to notice. This is all a spit ball of course, but it does seem logical....