I know this is an old thread but sounds like something I seel quite often at the dealer when it starts getting cold every year. The rubber lines start to get hard over time and the clamps just dont get tight enough to seal them properly. The most common place I found to leak is on the passenger side at the front of the head. Theres a place there where the rubber line starts to seep fuel and it drips on to the warm engine and evaporates pretty fast which is why you never see a leak. I have even seen the metal line start to leak a little in that spot as well. Easiest thing to do is want untill its reallly cold again and watch the fuel lines while you have someone start the car and if any start to get damp you found the leak.