It was the feds that told me that when I first started inquiring about getting a can.
I guess I should clarify: If you're not home it's OK to leave your NFA item locked up in your house. However, if you're the only registered owner of that NFA item and someone else has access to the item when you're not home, that's a violation of federal law.
So let's say you leave home and all your NFA items are locked in your safe. And let's say your wife opens that safe. That's a violation of federal law as the BATFE chooses to interpret it. That said, they're not in the habit of busting married couples for this and as a result most people don't worry too much about it. But people who are
worried about it, or just want their wife to be able to use their NFA item without them being present, just purchase the item through a trust and include their wife as a trustee.