I like using LR mobile as a gallery of my work, as well as an editing platform. I use a tablet that only uses WiFi, and during those time when I am not connected, my images in collections that are not "enable offline editing" seems to disappear after awhile.
I always assumed that once you sync's an image to the mobile devise, the image would stay around in cache. I even have trying running a slideshow when connected to make sure I have a large sync'd copy of the images, and even those images go away. Is the only way to ensure a collection will be available offline for just viewing is to enable offline editing for that collection?