This happened to me. The driver decided to drop me off in the bus lane (instead of the normal taxi/car lane) at an airport and encouraged me to get out quickly and drove off just as fast because we were in violation of airport policy. Very shortly after I noticed my phone was gone and was in his car.

I was able to contact him since I luckily had a tablet with data and I was using Project Fi but there was absolutely no good process to deal with this since it was impossible to get in touch with him.

Uber support was completely useless and in the end he returned it to one of their service centers. He never told me so and enough time had passed where Uber just recycles/destroys phones beyond a certain point.

I was absolutely livid about this. I only use Uber as a last resort now.