I flew out of IAD yesterday, and used the on-line barcode-decoder to confirm the '3' character indicating a PreCheck boarding pass. I'm not sure why this isn't just shown on the boarding pass with a big icon: is it supposed to be secret?
I went to the dedicated PreCheck checkpoint, and was the only passenger there at the time (mid-morning). Mobile-boarding-pass was scanned, got the 3 beeps, and I got through the checkpoint in a few seconds.
Note: I do not have any other trusted-traveler enrollment, just the 'checkbox' in the MP profile.