After a user approves an invoice request or requisition, they no longer are able to view the record in WebPortal.
A user's security rights define which invoice requests or requisitions they are able view. In addition to requests which require their approval (if users are set up to approve requests), users can either search for the Invoice Requests/Requisitions they create or requisitions created by all users. If you want a user to be able to search for Requisitions or Invoice Requests they did not create nor require their approval, the box must be marked for "Can view all Requisitions" and/or "Can view all Invoice Requests".
If a user's rights are set up so that they may approve requests, but not see all requests, they will be able to search for requests they create and see requests that require their approval at that time. Once the request has been approved by the user, they will not be able to view the request as it does not require any action from them at that time and they do not have rights to search for requisitions they did not create. Below is a screen shot of the Online Security tab of a user's profile. The arrow is pointing to the setting which would allow a user to see all Requisitions.