Left front wheel noise?

[on air]

I hand't driven my '02 Boxster for about a week and a half. Yesterday got in and as I was leaving my nieghborhood the left front wheel started making this loud scrapping sound - sort of sounded like metal on metal. It would lessen a bit if I stepped on the brake but it was still there. Car has 35,000 miles. I thought brake pad worn? -- but if it was that wouldn't the sensor light up on my dash as well prior to it getting this bad?

then a few minutes later it stopped and I couldn't get the sound to do it again. When I got home, I jacked up the car, and the wheel spun freely - no noise.

Could a rock gotten stuck somewhere it shouldn't? Any ideas?

thanks