Three nights in a row is the detail that gets me. One night you can chalk up to anything - pipes, a mouse, your own tired brain playing tricks. But three consecutive nights with the same sound, same location? That pattern means something.
Done a fair bit of driving through the night over the years and you learn to trust pattern recognition. Random noises are random. Patterns have a source.
Have you actually opened the closet while its happening? I know that sounds obvious but most people don't, they wait til morning when everythings quiet again. The immediate investigation is always more useful.
Also what kind of breathing are we talking - rhythmic and slow, or more irregular? That distinction matters more than people think. Anyone else had something similar? There's a thread from about 2019 on here somewhere where a bloke in Leeds had almost identical experience, turned out to be genuinely unexplained even after they ruled out the obvious stuff.