This happened to me last month in Halifax Canada, I wanted to board a transfer flight to Amsterdam, going through Boston. I was stopped as I had salts with me from a buddhist relic I couldn't exactly identify. The machine said it contained a primary explosive (silver azide). I was ordered to give my access code to my iphone and 3 suspicious screenshots were found I made 2 years ago about the Iran/Israel war. I said I'm interested in geo-politics and watched the news. I was barred from the US for life - it was said that it could have been much worse. When I wanted to go to the main airport hall a number of canadian police officers arrested me and drove me handcuffed to a cell. I was asked if I wanted a lawyer and they arranged a phone call. The lawyer said I would spend to nights in a cell and then she would come to prepare my case and was strongly advised not to talk to people before that. I was released from my cell 7 hours later that evening after a thorough and friendly conversation with the canadian special police. I can understand that security don't want to take any chances with people carrying supposed explosive powders on an airport, so the system was doing it's protective job. That day there was a threat alert I heard later. A week later I heard from the person who gave me the relic (I was on retreat) that the border guard contacted them and said it was a false positive (what I suspected at the time, but I didn't dare argue against the machine results).