I poured through the laws and talked to a lawyer about it, though this was years ago. It is illegal to inform a user directly in any way, they can't make you re-architect your system to provide them a backdoor unknown to the user.

also the wording can not mention 'warrant' or 'fbi' so it has to be something like 'third party access'

I have been meaning to do this as a 'project' with full legal advice etc. and suggest it to google, other cloud providers

Edit: found that rsync.net already do this, in a different way, as a warrant canary: http://www.rsync.net/resources/notices/canary.txt