While every aviation security authority has different policies for hand searching luggage, the TSA is often considered the benchmark of industry practice. This is because the TSA has the most employees, the largest budget and some of the strictest safety rules of any transportation security agency.
Despite this, in a recent investigation by the Washington Post in November 2014, a TSA official revealed a surprising statistic:
Only 4% of checked luggage was subject to a hand search by TSA employees.
Full text of the article is available here. This statistic has not changed substantially over time. Back in May of 2008, Sean O'Neill of Budget Travel obtained an official TSA response to questions readers had asked the publication. The following is an extract from that article:
We asked the TSA to respond to the many questions that readers posted to our recent item, "Has the TSA stolen from you?
Q: How many complaints are there a year from lost or stolen items?
...When an item goes missing from a checked bag, it is often impossible to determine where the loss occurred given that checked bags pass through so many hands. Remember, TSA has possession of the bag only long enough to screen it for explosives. Bags are delivered to TSA by the air carriers or their contractors and we return all bags to the airlines after screening. TSA never even touches the bag at the connecting or at the destination airport. We estimate that for every TSA employee that touches a bag, six to ten airline or airport employees and contractors touch the same bag out of the view of passengers.
Q: Several of the readers of our blog say that there's one large difference between TSA workers and airline workers. They say it is easier for a TSA worker to act alone when committing a theft of a passenger's goods because TSA workers often do inspections alone. Are the working conditions for TSA workers more conducive to acts of theft than the working conditions of airport workers?
...Because TSA screens every bag for explosives electronically, only a very small percentage of checked bags are actually opened by TSA security officers. Bags are only opened to resolve an alarm and searches are conducted often in public areas by well-supervised security officers who work in teams. At the end of every bag search, a notice is placed in the bag indicating that TSA needed to open the bag.
The full text of the above article can be seen here: http://www.budgettravel.com/blog/theft-from-baggage-the-tsa-reponds-to-our-readers,9834/
In light of the above facts, every traveler needs to ask this question: Is it really worth the risk of travelling with unsecured luggage?