Smart Collection in Camera Raw not finding files

One of my great frustrations with Adobe Camera Raw is with the "Smart Collections" feature. I put each year's images into a folder and then create subfolders for each day I shoot. I edit the images and give a star rating to the best ones. I then create a "smart collection" to find all images rated 3 stars and above. In a given year I may have 500+ stared images but only a small number appear in the collection. If I then go through each folder and open it and look at the stared images, they then appear in the collection. However, if I don't do this for some time, the "collection" doesn't find all the images again. Are they kept in a cache that is deleted?