a hidden document - covering device for a scanner , especially for a scanner built in the case of a computer in which a device is used for holding the document on a moving tray during scanning process . the document - covering device always hides itself in the case even when the tray moves out . during the scanning process , the document for scanning is firmly pressed onto the tray to prevent from curling or sliding . after scanning , i . e . when the tray moving out , the document - covering device will release the document for easy loading and unloading .