According to a recent press release form the ABA:
Lawyers who receive electronic documents are free to look for and use information hidden in metadata – information embedded in electronically produced documents – even if the documents were provided by an opposing lawyer, according to a new ethics opinion from the American Bar Association.
It goes on to explain what such metadata might reveal:
The ABA committee noted metadata is ubiquitous in electronic documents, and includes such information as the last date and time that a document was saved and by whom, data on when it was accessed, the name of the owner of the computer that created the document and the date and time it was created, and a record of any changes made to the document or comments written into it.
“Other types of metadata may or may not be as well known and easily understandable … Moreover, more thorough or extraordinary investigative measures sometimes might permit the retrieval of embedded information that the provider … either did not know existed, or thought was deleted,” said the opinion. And while the opinion said most metadata “probably is of no import,” it added that the metadata can sometimes reveal such critical information as “who knew what when,” or negotiating strategy and positions. (emphasis added)