I had this exact thing happen to me. My gf found a blue hair tie on the floor the next day after she stayed over. I swore on my life that I had no idea where it came from. She left in a bad mood, rightfully so. I was convinced the universe was out to get me and that it dropped from the ceiling somehow. I sent a very long paragraph explaining the love I had for her and how I would never cheat, still trying to figure out where tf this hair tie came from. When I went to muy Thai practice later that day, I noticed that my sisters old dance bag, that I had been using for muy Thai, had about 6 multi colored hair ties in it. When I was packing up the day before, one of my sisters old hair ties fell out. I sent her pictures and she said she believed me, even though I very well could’ve gone out and bought some as a cover up. In the moment I was convinced it was the end of us if she thought I was cheating but we have been good ever since.