I've run in to a similar problem with our archives. For now, I'm making a note under Provenance and/or Acquisition to the effect that no deed of gift or loan agreement is on file. I'd be curious how others handle this situation!

Hi Siony and Victoria: 

When the copyright/legal ownership of the item is unavailable or unknown, the item is known as an "orphan". I think Victoria's suggestion is a good one, I would create a collection file record for the item as a place holder to reflect that the origin and ownership of the item is unknown. The use of this item in publications or publicity may need to be restricted, based on the policy of the holding institution. 

There are a number of ways to handle this, and some professional organizations in the library/archives field have released statements about their best practices. It is something that everyone deals with, and is seldom a problem until the donation is put into question and subjected to scrutiny, or the item is of great historical value and the institution wants to publish or exhibit the item. You can read the SAA position from 2009: 

http://www.archivists.org/standards/OWBP-V4.pdf 

For published works and more on the library perspective: 

http://www.arl.org/publications-resources/position-statements/term/summary/66

Best wishes, 

Jennifer