Passport Control detected something on me

I was recently coming back to the US from Iceland and was “randomly selected” for additional screening. They swabbed my Kindle, phone, luggage, etc. and my Kindle came back with some type of alarm. They swabbed it a few more times and it kept testing positive for SOMETHING, although they refused to tell me what. They brought in a couple more people to ask me a bunch of questions like “how long have you owned the Kindle”, “has it ever been repaired”, “has anyone else used it”, “has it been in contact with any chemicals”, “what have you done in the last 24 hours”, “tell me everything you’ve done while you were here”, etc. They also took all of my belongings out of my luggage and inspected everything before eventually sending me on my way, although they seemed very suspicious of me and put a sticker that said “RED” on my passport. The whole process took maybe 30 minutes, but I’m very curious what they detected on my Kindle, since I legitimately had nothing to hide. Going through customs when I arrived to the US was super quick and no additional questions came up, so I removed the sticker from my passport. Should I throw my Kindle out?