Pre Check determination is made at time of check-in. It's not made in advance. It's quite similar to how the CAPPS (Customer Assisted Pax Prescreening System) worked. When you push the check-in button, the airline runs a routine that looks at your PNR, a gov't database, and other things (I can't discuss) to see if you meet the criteria for Pre-Check. If not, then you don't get pre-check.

Random statistic on CAPPS: On 9/11 it flagged a majority of the hijackers as selectees, but unfortunately as we all know, that wasn't enough. At the time the system was only designed to handle PPBM (Positive Pax Bag Match).

CAPPS II never made it to fruition, the TSA got more interested in playing with nudie boxes.