Your days of unexpectedly finding a "TSA PreCheck" mark on your boarding pass — without actually being a paid member of the program — are numbered. "Starting early February 2017, TSA will significantly reduce access to [TSA PreCheck] expedited screening for non-enrolled travelers," Transportation Security Administration spokesman Mike England told CNBC in an emailed statement. The TSA has not put out a formal announcement, but United Airlines alerted its frequent fliers to the upcoming changes in an email last week.

Since the program's launch in late 2011, the TSA has granted some nonmember travelers access to PreCheck lines on a flight-by-flight basis, England said. The change will affect "very few" travelers, he said, and the agency does not anticipate any effect on lines.

"This is part of the natural progression of the [TSA PreCheck] program," he said. "In the future, we intend to only have enrolled or pre-vetted passengers, or those screened by K9s, in the expedited screening lanes."

Limits on access

The TSA has previously warned of curtailing nonmembers' access. In a 2015 blog post shortly after the PreCheck program hit the 1 million mark, the agency wrote, "As more and more travelers obtain KTNs, soon, travelers without a Known Traveler Number or KTN, including those who previously 'opted-in' via a frequent flyer program, will notice a reduction in the frequency in which they are chosen."

Earlier this month, the agency announced that PreCheck had topped 2 million members. Just last week, it said the program would expand to 11 new airlines, including Spirit and Virgin Atlantic, for a total of 30 airline partners.

"That may be very well why they want to make sure nobody who isn't in the PreCheck program is going to get in line," said Ed Perkins, a contributing editor for SmarterTravel.com. "Those new partners are going to increase the volume."

Value for frequent fliers

For frequent travelers, applying for a trusted traveler program such as PreCheck or Global Entry is a good value, Perkins said. The former works out to $17 per year; the latter, $20. Plus, there are plenty of ways to avoid paying the application fee, he said.

Passengers make their way in a security checkpoint at the International JFK airport in New York October 11, 2014. Eduardo Munoz | Reuters

Several credit cards offer reimbursement, and business travelers may be able to get their company to cover the cost. Several airlines also allow travelers to use miles to cover the application fee, although advocates say you're better off keeping them to use for travel. There's also a private trusted traveler program, Clear, said Perkins, which administers its own screening lines in 22 airports nationwide.

Shorter lines for some