passenger airbag light problem (NOT stuck on)

[ersheff]

Hello.
I've had my 2010 Yaris for about 5 days now.
The passenger airbag light is confusing me.
When I start the car, all of the passenger lights (airbag on, off, and seatbelt) come on and then turn back off, so I know the lights themselves work.
When someone sits in the passenger seat, the airbag on and seatbelt light come on as expected.
When no one's sitting in the passenger seat, there's usually NO lights on. Occasionally I see the airbag off light, but there doesn't seem to be any rhyme or reason as to when it comes on (aside from the fact that the "off" light does not come on when someone's sitting there, so it's not totally malfunctioning).
Any ideas?