New evidence revealed this month suggested that a 1990 art museum heist was an inside job, RadarOnline.com has learned. A secret signal sent out to the suspected thieves was also revealed. Flash back to March 1990, and the Isabella Stewart Gardner Museum in Boston fell victim to a daring art heist when two thieves disguised as police officers made off with $500 million worth of artwork – including pieces by renowned artists like Rembrandt and Vermeer. The heist, which was believed to be the largest of its kind in history, left the museum reeling with nothing more than empty frames that still h…