You are being unreasonable.
" I'm in my en-suite, in my bedroom, on a different floor of the house while he's downstairs doing breakfast fir the children. I'm not expecting him, or them, to come back upstairs"
I had a shit in a dense wood, about 30 miles from the nearest civilisation. I had already walked for about three hours without seeing anyone, so certainly wasn't expecting anyone to appear, when an entire Outward Bound group suddenly appeared, about 40 yards awat, in my direct line of vision.
I think their appearance was fairly unexpected, but even then I would have been unreasonable to blame them!
Had I been in an open doored bathroom which led directly off a bedroom that I actually shared with someone else, I would have been totally unreasonable to expect them not to come into their bedroom or open doored bathroom.
Unless you habitually poo in front of each other, I think your DH was entirely reasonable to assume 'closed door = possible poo' 'open door = teeth / hair / make-up etc'.
Are you looking for things to fight with him about, ThisllOutMe? I don't mean that unkindly ... it could be that deep down you feel it's the end of the road, so you are trying to justify that feeling.