Do I have a fuel leak?

I have a 2000 Forester. I noticed lastnight that my gas gauge seemed to be dropping quicker than usual. It went from half a tank to a little more than a 3rd of a tank in a matter of 20 miles or so. I was perplexed, but I thought maybe it was my imagination. I filled up with about 1/3 of a tank left. My car will usually sit on FULL for about 40-50 miles before the gauge starts to fall. After another 20 miles, it had already gone probably another 1/8 of a tank. By the time I got home (a total of 40 miles after fill up) I only had 3/4 of a tank left. I was driving between 70-80 mph on the interstate, but that's actually slow for me and I still get far better gas mileage than lastnight. There was never any smell of gasoline. After I got home I got under the car to make sure I didn't see, hear, or smell any gas. I didn't smell anything, but it was dark and I was using my cell phone as a flashlight. I thought it had to be leaking gas and fully expected my tank to be close to empty by the morning and a puddle to be under the car. I was pretty surprised to see nothing, still smell nothing, and when I cranked my car, the fuel level hadn't decreased. I drove around for a while trying to find some place to take it, but being New Year's Day, everything was closed. By the time I got home I was down another 1/8 of a tank. I'm taking it to the dealer tomorrow, but I'm just wondering if anybody has any ideas on what this might be. It's tough being a female taking your car to be repaired because they always assume you're an idiot and I like to be as informed as possible so when I go in, they don't try to pull one over on me or dismiss me.

Any chance you could have just bought bad gas? I find that if I end up having to buy gas with ethanol in it (even though Subaru says you can't put anything in over 5% sometimes there just isn't a choice) that I get much worse mileage than when I can get the non-ethanol gas.

As for dealing with dealerships as a female, TRUST ME, I understand. Good luck!