I have similar experiences with my Runners. First few days I think one has disappeared until I realize they are sitting on a nest full of eggs. Then a few days later I think I must have been mistaken because they spend so much time off the nest. Then suddenly they are serious. Quietly defend the eggs for most of the day with a few intense foraging outings, and then the last few days they hunker down, hiss/growl, and won't move for anything. Mine will leave now for about 5-10 minutes when they are desperate, but if they think I'm lurking they won't budge, because they seem to know I might borrow an egg or two for candling.
I thought mine might be hatching this weekend, but it seems those earlier, not so serious days, delayed the process by a few days. I'm a bit of a nervous Nellie hoping to have everything in better shape for ducklings this time around. Last Spring I thought I had my 'Moms' and their ducklings fairly secure from the other ducks, but had NO idea how tiny the ducklings would be and what they could squeeze through. They were confused and off seeking mothering from my other ducks within minutes of being in the open run (in spite of me thinking they'd be restricted to their own smaller pen) and of course the desperate Mom's couldn't get at them. I managed to rescue everyone eventually and 'herd' them back inside until I got more protective fencing so the 'hardware cloth' from the start is a good idea.
Have fun!!!!!