Upvote:4
Besides members of Trusted Traveler programs (such as Global Entry, NEXUS, and SENTRI), some people are permitted to use the TSA Pre-check lanes as part of other TSA programs, such as managed inclusion. For more information, see: https://skift.com/2016/02/16/tsa-precheck-lanes-wont-be-members-only-for-a-long-time-still/
Also, only certain airlines participate in the TSA Pre-check program, so when you are not traveling on these airlines, you will not be eligible for Pre-check, regardless whether you are registered or not. As of July 2, 2016, the airlines participating in TSA Pre-check are:
If you are not a member of a Trusted Traveler program, then you likely will not be granted access to Pre-check on other airlines, particularly if you do not input your Known Traveler Number "KTN" into the other airlines' systems. TSA advises "Please check when booking that your KTN has populated the appropriate fields in order to qualify for expedited screening."
Source: https://www.tsa.gov/tsa-precheck