Thanks for the reply.
I've done some research and the closest I've got to an answer is a thread on Google dating back to 2011
productforums.google.com/forum/#!topic/docs/wmgpk9N8LrU
It would seem that once a document is viewed on Google then it may become cached and indexed and subject to Google's T&Cs regarding privacy i.e. they can use it and make it public.
Of course I can turn it off - but then there is no viewer functionality in eDocman which is one of my main reasons for wanting to use it.....