Okay, I just went outside and tried this... Interesting results...
First, my setup:
A. 2015 Model S with AP1. Configured for Auto-folding mirrors, Auto-presenting handles, Auto-lock on walk away.
B. Car was in garage, locked, mirrors folded, handles in.
C. I was in the house (approximately 30 feet away).
Second, my actions:
1. Removed battery from fob. Pressed all buttons multiple times to drain residual energy.
2. Walked into garage from behind car, pressing buttons. Nothing happened.
3. Walked around car, from the back, to the driver's side, around front, to the passenger's side, pressing each handle. Nothing happened.
4. Placed the fob on the windshield, just above the passenger wiper blade, and the mirrors folded out, handles presented, lights came on. Note that I did not press any buttons or handles.
4a. My assumption is that the car was executing the normal presence detect as if I walked up to the car.
5. I got in the car, pressed the brake pedal, got the warning about no key in car.
6. While still holding the brake pedal, I positioned the fob under the 12V outlet and the car started.
7. I then pressed the lock icon on the screen, got out of the car, and closed the door.
7a. The handles retracted, but the mirrors did not fold in. The car was locked.
8. I then walked back around the car checking all handles. The car was locked.
9. I then went back to the windshield with the fob, but no matter where I placed it, the car would not unlock.
9a. I did try to push on the handle the whole time I was trying to find a spot which would unlock the car.
10. I used my phone to unlock the car, I got in, pressed the brake pedal, got the warning about no key in car.
11. While still holding the brake pedal, I positioned the fob under the 12V outlet and the car started.
12. This time I put the car in "D" and then back to "P". I hit the lock icon, and got out and closed the door.
12a. Again, I could not use the fob to get back in the car.
13. I went and put the battery back in the fob and used it to fully lock the car, which also folded-in the mirrors.
14. I took the battery back out and went to try again. It did not do it automatically, like it did the first time.
14a. I had to keep moving it around while pressing the handle, but finally did get it to unlock.
Moral of the story... it's finicky, but does appear to work with some persistence.