The Dragon, a massive oil tanker flying the Liberian flag, is supposed to be floating somewhere off the coast of France, according to its last GPS signal.

Instead, it's currently thousands of miles away in Venezuela where, under contract for the Russian state-oil giant Rosneft Oil Co. PJSC, it loaded 2 million barrels of oil, according to data compiled by Bloomberg and shipping reports. How's that possible? Because the ship's transponders were turned off before it slipped into Venezuelan waters, the data show.

The practice of oil tankers turning off their location signals has increased in the past month, according to shipping data, after the U.S. went after a Chinese-owned shipping company it said was moving crude for sanctioned Iran. The U.S. is seeking to squeeze the Nicolas Maduro government in Venezuela by starving it of oil revenue. But more and more tankers appear to be using the technique to avoid penalties, helping give a boost to Venezuelan crude output that has plummeted since the U.S. imposed sanctions.