2007 subaru imprezza tr i have 500 miles on it and sometimes when i drive the airbags sign comes? any ideas?

could it be like in many cars, the airbag sign to tell you that the airbag is disabled in the passenger seat?
When the passenger seat thinks there might be a child sitting there it disables the airbag. So if you have a bag on the seat, it might think its a young child.
That is why the airbag light is on.

It comes on at start up to inform you it is working normally. If after a few seconds, it does not turn off, or it comes on while driving, then you should take the car to the dealer to have it serviced.

The full explanation can be found in your owner is manual (or at least it can for my 2005), but that is pretty much what it says.