Does anyone know how the car "senses" that one of the tires lost pressure when the car has runflat indicator, but no real TPMS? Is there a device in the tire similar to a TPMS or is it through some other mechanism?

 

Its always the way on most forums that someone takes the time to type how someone can google a question when they could have answered it in the same time
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It goes on the tire rotation, a flatter tire will rotate more and the speed sensors pick it up.

 

Your car does, in fact, have a TPMS sensor on each wheel. Gross differences in pressure will be reported as a red or yellow indicator on the dashboard with a warning to stop and address the issue.

 

From my understanding TPMS and Runflat indicator are commonly confused as being the same thing, as the internet searches bring it up as the same stuff

TPMS = Tire Pressure Monitoring System will let you check what pressures each tire has and will also alert you to tire deflation, Each wheel has a sensor in the tire valve, TPMS will let you check your tire pressures in real time.

Runflat indicator will only tell you that a tyre has lost pressure, not all cars have TPMS sensors, it depends on the level of the car or options.

If it comes up in your dash as a indicator with a top view of a car, showing your actual tyre pressures of all 4 tires ( and a little yellow flat tire icon if you have a flat tire ), you have TPMS and each wheel has a sensor that relays the pressure to the car.
Pretty sure If it comes up in your dash with "Run flat indicator active, Restart with OK" you have only a runflat indicator and it uses the speed sensors to determine if a tire is flat by the rotations of the wheel and will alert you to which wheel is flat.

Generaly with TPMS your display will have the pressures in white if they are ok, yellow if a little off, and red if they need attention.

 

A runflat tire is not an airless tire. Under regular use it should have in it the same tire pressure as any regular tire. If your tire runs low or out of air, then the car will alert you with a low pressure warning. From everything I've heard running these tires for 5+ years, you can run it with lower than normal pressure for a short amount of time and keep the tires. If you run these tires without any pressure though the tire will need to be replaced.

 

And, depending how much wear is on the tires on that axle, the one on the other side may have to be replaced. I understand that 4Matic doesn't play well with a new tire on one side and an old on the other. It's possible also that new tires on one axle won't get along well with worn tires on the other.

 

This is the correct answer, my car does not have tyre pressure sensors yet it can monitor wheel speed via the abs and detect if one wheel has a different effective rolling radius and hence speed due to the tyre shape (only flat on the bottom). I recently had this and found one tyre was indeed low in pressure, after pumping up the tyre the system required me to reset it and the error message disappeared. Higher level cars than mine have pressure sensors on each wheel that monitor actual tyre pressure rather that wheel rotation speed - you gets what you pay for.