I have never seen the "LLL" on any of my AA flights since I registered for pre-check, although I fly AA quite frequently. The thing is I didn't know my pre-check information transferred to another airline outside AA. I was surprised when I was going through the security checkpoint for a Delta flight and the TSA agent told me I should go through the Pre-Check lane after scanning my BP. I stayed in the regular lane explaining to the TSA agent that my traveling companion was not registered for pre-check and we were travelling together.
I am curious if one registers for Pre-Check through AA, does it apply to all other airines (UA, US, DL, WN, etc)?