So can someone tell me what THIS indicator means? Has something to do with the back seat. It's turned on when I start the car, and turns itself off a minute or two later.
No one is sitting in the back seat, so it isn't as if someone back there doesn't have a seat belt on. There are a couple of towels sitting on the back seat, though. It is
annoying that the CX-5 manual doesn't seem to have a MAP of what indicators appear where.

 

You don't indicate what is the year or the trim of your CX5. I have not seen this before, but just looking at it, it appears to be a warning that the backseat seatbelts are not fastened. Since it comes up when you start the vehicle and then shuts off, you are probably seeing the system test that lights up multiple indicators on the dash at startup and then shuts them down when the tests are completed with no errors.

This suggests to me that you have a late model CX5. In the last few years several states have passed laws requiring rear seat passengers use seat belts. I know Indiana is one of these states. Mazda may have added this indicator to accommodate such states or it may be that the vehicle was prepped for use in such a state. My suggestion would be to have someone sit in the back seat without a belt and start the car. If the appropriate light does not go out, then have them fasten the belt and see if it does. That will probably answer your question.

 

2025 Base model S.

Yes, the back seatbelts aren't fastened, because no one is sitting there. As far as I'm concerned it IS an error if it lights up when no one is sitting there, even if it turns off after a little while. No, it's not just turned on at startup. It's on for a minute or more. That means I'm looking at a notification alarm when I'm backing out of my driveway and driving up the street. What's the purpose of that?

 

In this age of nanny-statism, the purpose would be to make you check you rear seat passengers' seat belt use. On my MX5, the passenger seat light lights at startup and then goes out about ten seconds later if no one is seated there.

Have you had someone sit in the back and fasten a seatbelt to see if the status changes?

 

Yes, I ought to do that, but that doesn't forgive the warning light being on while I'm well underway. For a minute or so, I guess I'm being reminded to buckle in the people who aren't sitting in the back seat. There are many other warning lights that don't go on when I'm underway.

 

The seat belt warning light comes on in my CX-5 at start up.
The newer models have indicators for both front and rear.
They go off when the belt is fastened and/or the vehicle is underway, and the safety system realizes no one is in the seat.
You only have an issue if it stays on and doesn't go off.
Unfortunately, any daily ride will show, many drivers need to see this warning to comply with laws and even then, many still don't.
Often being severely injured and/or dying because they refused to use safety equipment.

 

Like others have said, it's nothing to worry about. It goes away once you start driving. Each seat will light up green if the buckle is closed, and a specific seat will flash red and beep if someone is sitting in the seat without their belt on. The lights will turn off once the car detects nobody sitting in the back seats.