I recently purchased a used 2017 E300 sedan with luxury trim.
Upon attaining 50K miles, the following warning symbol(s) appeared on the dash screen near steering wheel (see picture below.)
What do these symbols mean? (A blue box with an enclosed white letter P; an overlaid symbol that looks like a movie projector or ?)
Is there a comprehensive index for future reference?
Thank you!

 

I believe that indicates the parking cameras are active. Likely because you were moving very slow or stopped but still in gear.

 

The warning symbols won’t disappear. How do I clear them?

 

On page 8 of your owner's manual there should be a list of warning symbols - https://www.mercmanuals.org/mercedes-w213/

That said, the symbol you see here is not a warning, it's informational. This symbol, as explained by @L1Wolf, is informing you that the parking cameras are active, as your car is equipped with the Parking Pilot system that lets the car park itself. This symbol appears because the vehicle is traveling below 22mph, and is looking for parking spots with the cameras. You cannot make this symbol go away without increasing speed above 22mph. It is not a warning, it's merely informational.

 

The symbol won’t disappear even when driving at speeds higher than 22 MPH.

 

It never disappears at all, no matter what speed you drive? It should disappear in park as well.

 

Check that the camera switch on the left side of the console is operating correctly.

 

The car is telling you that you can use the Park Assist if you want to.

 

Strange that it should only show up at 50k miles. Was there anything done at that time? Any kind of service by the dealer? How long have you had the car and what was the mileage when you got it?

 

49,460 miles when purchased. The symbol did not appear in all my city/highway driving until after about 50,025 or so. And it intermittently reappears even at speeds >50 MPH.

 

It's not a warning light. It's just telling you that the feature is activated.

When you see the P, the vehicle is scanning for a parallel parking space. If you turn on your turn signal (blinker) slowly drive pass an open spot. If you see a white arrow next to the P when you're about even with the car in front of the open spot, stop, and place your car in reverse. You'll be prompted to press OK to begin park assist.

I also believe this is for your sensors. You can turn this off on the center console. I forget what button exactly, but I can send you a picture of it in the morning when I get inside my car.

 

So why does the operator’s manual say (page 186) that parking assist feature is operational up to 22 MPH, after that the symbol drops from the dash screen?

On my vehicle the symbol reappears intermittently at speeds much greater, such as 50 MPH.

Does this sound like a problem with the engine computer or ?

 

It seems like the Park Pilot system isn't getting correct information. If you have a code scanner, I would look for any codes that are related to this system. If you can't find anything and it bothers you, you could take it to the dealership, but I doubt that it's worth ten minutes' worth of their hourly rate to solve something that doesn't create any actual issues.

 

I would say no, but it could be a problem with your sensors. I am more mechanically inclined than electrical. I don't want to give any wrong answers, so I would plug in a computer like @ArchAngel2190 mentioned.

I can also test in the morning and check if mine does the same thing. But I believe that mine goes away above 15-20 MPH.