The Los Angeles Times recently had an article about the accumulation of pocket change at the security checkpoints (over $376,000!) and the Transportation Security Administration's use of those funds. As the article explains, current federal law allows the TSA to keep all amounts left at security checkpoints "for the purpose of providing civil aviation security" (see here for the applicable law).
According to the LA Times, Congressman Jeff Miller (FL-1st) wants to change that law by providing that such unclaimed funds would be donated to the USO. That legislation (text here) is currently pending before the House Subcommittee on Transportation Security.