When returning from overseas on a non-participating airline the only way I know to get PreCheck for the US domestic legs is to ask for new boarding passes once we hit the US and switch to a participating airline. For us most of our travel outside the US is on AA or a OneWorld partner. If the return flight is AA metal from start to finish then we've had no problem getting PreCheck on our boarding passes from the get go. If it's say BA to the US then we've had to get the boarding passes reprinted. I've always assumed non-participating airlines, even ones who are partners with participating airlines, simply don't have access to te PreCheck system nor should they.