Customs agents are blasé about catching businesspeople, but they were amused to see an American family of four, including two young children, in a discreet corner of a railway station near the Swiss border, dividing €600,000 among themselves. The money was seized when they boarded a train, according to Mr. Bock of the customs trade union.

Cash seizures by French customs agents have soared over the last decade even as budget cuts have thinned the agents’ ranks by 25 percent. The total for the first quarter of 2013 was up sixfold from a year earlier, to €103 million, most of it from a man who tried to drive into France from Switzerland with €86 million in bearer bonds, which are tantamount to cash. On an average day in 2012, French agents seized €300,000, 50 percent more than the 2011 average, according to government figures. And the customs agency estimates that it catches only 5 percent of the undeclared cash crossing the country’s borders.

The precautions are growing more elaborate, and the finds more eye-catching. In February, inspectors on the fast train between Zurich and Paris stopped a Spanish traveler who was carrying €1.8 million ($2.5 million), made up entirely of 500-euro notes. Those bills, the largest denomination in circulation, have come to be nicknamed Bin Ladens for their association with money laundering and illicit transactions.

In Italy, cash-sniffing Labrador retrievers and German shepherds helped the financial police who prowl the country’s five main airports and its northern border to nearly triple their seizures of cash in 2012, to €124 million. The authorities were close to exceeding that figure this year by mid-autumn.

Sergio Callipo, national secretary of the Italian customs agents’ union, said that much more cash slipped through. “Millions of passengers pass every week, and some officers and one or two dogs are not a real deterrent for smugglers,” he said. “We would need an army, and we are just sentinels.”