Flying out of IAD tonight to AUH on Etihad. Purchased my ticket on Expedia and entered my Global Entry KTN in my Expedia profile. I got Precheck on my boarding pass. The agent said she would have been able to enter my KTN had I not gotten Precheck.
Interestingly, my companion got Precheck as well. We're on the same reservation, but he doesn't have a KTN and has never gotten Precheck. The experience just convinced him to sign up for GE.