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Here's a bug similar to random Barbarian events occurring at a sieged fortress:

I was sieging a fort, and I wanted to get rid of pesky scouts, so I patrolled. It's probably irrelevant, but I also stormed the fort the same month with one unit, just to peek inside. Anyway, I got a message about finding a sneaking enemy, but strangely there was no battle that I could click on.

The next month I stormed the fort and took it ... only to find that I no longer controlled the province. It was now owned by the scout I had uncovered the previous month, when I patrolled.

I had armies moving into the province the month I stormed, but since they were on 'move' and not 'move and patrol', they just ended up inside the fort, instead of fighting the scout. Not that it's possible to 'move and patrol' to a fort you're sieging.

Next time I discover a sneaking enemy while sieging, I'll be sure to leave a small force still patrolling when I storm a fortress. That way the scout won't capture my zero-PD province.