I've had some mysterious flats over the years, just keep analyzing is my best advice. Might also check the area around the valve for when it's stressed a bit, might not be evident when just inflating outside of the tire (i.e. sharp edges around the valve cutout of the rim). I've had some stuff stuck in the tire where it just wasn't visible or felt with fingers until turned inside out and stressed. You could also try a different tire with the same tube, and similar combinations to narrow it down. Good luck.