I would try and shut her in one room and set the nesting box there so that you can make sure you can definitely spot the little ones when they are born if she has them whilst you're away/asleep.
Hold the kitten to make sure it is latching, if she is disinterested then try to squeeze some milk out first leaving a drop on the to stimulate the kitten's smell senses. Uf the kitten is not gaining any weight then you should supplement immediately !let me know how the rubbing goes, i had to...
I would suggest to get a cotton bud and put a bit of warm water on it and start to rub the other nipples to get them to become active -sometimes the skin on the nipples is dry or the milk channels are blocked - this helped me to get all 8 of my cat's nipples to work, even when one was inverted...
Yeah i would take that as a sign, she could give birth tonight or in a couple of days, probably 2 or 3 would be my guess, speaking from experience.have a look whether you can see/feel milk building, just dont press too hard not damage the kittens.