ok my thoughts on this is that he actually figures it out from JIMIN! like Jimin mentions offhandedly once about how it irks him tae always leaves the window unlocked and jk is like wym always...I think he thought that was a possibility in the back of his mind but never fully believed it. but at that point he and tae would already be together and I HOPE in a much healthier place. so instead of him spiraling about it I think he'd be able to see it as a sign of how much and how long tae's loved him