hidden lien againt property

If we buy a trustee sale property out side of court, usually it takes 15 days to get the trust deed in California. What if someone record a lien after you paid and before you received the trust deed?

See civil code 2924. If you record the deed within 15 days it is considered to have been effectively recorded 8am the day of the auction.
Beyond that it is still relatively irrelevent as junior liens are typcially wiped out by the trustees deed anyway.