Although you may not intend it—and whether it's been earned or not—there's a good chance you're leaving cash tips for the Transportation Security Administration at the airport.
In their rush to get to their destinations, travelers left $638,142.64 in coins and currency in the bins and bowls at airport checkpoints in 2013, according to TSA data. That was almost $107,000 more than what passengers left behind in 2012, and more than $150,000 than 2011.
Read MoreSaving time and money at the airport
A spokesman said it appears that the growth in left behind spare change will continue, a suspicion borne out by 2014's figures: left behind change surged to nearly $675,000 last year, new TSA figures show.
So what exactly happens to all that money? According to federal law, TSA gets to keep it and spend it on anything the agency determines will help civil aviation security.
However, if you're not interested in inadvertently tipping Uncle Sam, you can empty your pockets of small change before you get to the checkpoint. The best place for it is in a pocket or pouch that fits inside a purse or carry-on.
Still, some airports have alternative ideas. In Denver, Phoenix and Columbus, Ohio, there are pre-security collection boxes by the checkpoints, where travelers can donate spare change to local nonprofits.
Denver International Airport started the trend in early 2013 with change collection containers placed before several checkpoints. In two years, the airport has collected over $170,000 in spare change to support homeless programs through Denver's Road Home charity.
Last spring, Fifth Third Bank set up three "Empty Pockets, Full Plates" collection stations near checkpoint entrances at Ohio's Port Columbus International Airport. In the first six months, the spare change raised about $1,000 to support the Mid-Ohio Food Bank.
Just before all those Super Bowl fans came to town, Phoenix Sky Harbor International Airport kicked off its spare change collection program, with boxes set up in front of several security checkpoints. During February alone, more than $1,000 was collected to help fund USO operations at the PHX airport, said airport spokeswoman Heather Lissner.
And in Sweden, travelers with spare change can donate to the Sweden Red Cross by playing video games at the airport. Custom-made consoles recently installed at Stockholm Arlanda Airport and Göteborg Landvetter Airport offer travelers the opportunity to pay classic arcade games Ms. Pac-Man, Space Invaders and Galaga, in exchange for coins in any currency.
—By Harriet Baskas, special to CNBC.com. Baskas is the author of seven books, including "Hidden Treasures: What Museums Can't or Won't Show You," and the Stuck at the Airport blog. Follow her on Twitter at @hbaskas. Follow Road Warrior at @CNBCtravel.