A car is just a car. A car with SOS next to it is someone that needs help.

A car in a remote dry riverbed is still very unusual on its own

You can do as you like. Me, I'm going to tramp out the international distress signal in letters big enough to see at 10,000 feet. "Hey, there's an SOS! Let's check it out!" And I'll be back at the ranger station thanking them for saving my behind.

You, you might get lucky when the ranger says "hey, wasn't that same car there 3 months ago? Let's check it out!" and recovers your desiccated body.

Tramp out how? You'll find little terrain that you can alter. Many trails are faint (as in a novice will likely not see them when they're right there) despite nothing erasing them for ages. There's minimal vegetation already, nothing that will show to a passing plane.

I would consider a signal mirror a last resort as using one means you can't take shelter and will cook all the much faster. Smoke is more of an option but I would much prefer to save my gasoline for running the AC than for burning a tire. If I had some other fuel source fine, but it's such a low probability scenario it's not something I'm going to prepare for. My safety standards out there is never to be farther from asphalt than I can hoof it back if need be.