Welcome to the Forum!
Hogs in the wild do tend to move nests from time to time – not least, because unless they do, there may be a build up of external parasites. The males are probably likely to move nests more often. But now there are so many hog boxes around, it seems that some hogs are choosing to stay put for longer. Sometimes making it very difficult for their ‘landlords’ to clean them out at the appropriate time! But, especially in the summer, some hogs just make a very temporary nest for a day or a few days. i.e. I have had them in my feed boxes before now and they will just curl up there with very little or no bedding at all.
I’m not sure I totally interpreted the second part of your post properly. Not sure if you meant you hadn’t seen the hog for 3 weeks, or it had been there for 3 weeks. But it isn’t always easy to work out whether a box might be being used as a birthing nest. If you have hogs visiting, there is also the chance that a mother hog will bring her young ones from another garden to get some food in yours, once they are big enough. Fingers crossed!
Good luck and happy hog watching!