If the air bag was missing there would most likely be siginificant damage to the dash and would only have gone off in a impact which would be noticeable on the bodywork.
I had the same problem on my clio for ages when I first bought it, mainly coming on when my mates got in the back, but after a few jiggles over the space of a couple of weeks it went off and hasn't come back on again, fingers crossed.
If you are really worried it'll be a trip to Renault or someone with an
ECU code reader to check it out.
Jake