When a user opens an attachment from within Outlook, the file is saved to a hidden location on the workstation they are using. Sometimes, an end-user would edit this copy of the document, save it, and then when they go to re-open it they can’t find it.
The hidden folder location is stored in the Registry under the “OutlookSecureTempFolder” key. The folder name is randomly generated. Search results do not bring up the location. Below is a quick-list of the registry entry you want to look at to give you the full folder name: