Hello @Alasdair3,
There is also the possibility that your three day notice was clicked, but that it turned off at some point on its own. I have seen people comment here that Instant Book was turned on when they knew they hadn't done it, blocked dates opened on their own, and other weird changes have happened as well. Perhaps the airbnb ghostie? Who knows..
My solution is to run down my listing weekly or so just to check all my settings/prices/availability. Doesn't take much time, and better safe than sorry.