When returning an AirPad for repair, replacement or return, check the DCIM/.thumbnails folder for cached entries. A Factory Data Reset does not clear this folder. For privacy reasons the .thumbnails folder should be deleted after a reset as it contains viewable jpg images and you may want to keep them private.
Also note that the jpg thumbnails in this folder may not even be related to photos put onto the tablet. When an USB drive is plugged into the USB port all images on the drive are cached into the .thumbnails folder. If a USB drive with 400 images is plugged in to copy over 5 photos, there will be 400 entries made into the .thumbnails folder. If two days later it is desired to copy over 5 more photos from the same drive the process is repeated so now there may be at least a total of 800 entries in the .thumbnails folder. Entries can accumulate fast taking up resources.