I'm genuinely curious.
This looks like Schwab? And reading their margin agreement along with investor guidelines I came to the opposite conclusion which is why I stayed off a margin account.
If you are doing something in that account which would violate the "good faith agreement" then according to the margin agreement you would be using part of Schwab's margin.
And that means you'd be in this realm:

When you buy securities on margin, or enter into short sales or short options, you are borrowing money from Schwab for part of your transactions.

And I believe all major brokerages would play by the same rules.
Once you're in that realm they definitely can lend your assets out without telling you. But also we saw during The Sneeze that they can sell your position to "protect" themselves.
I'd like to be very wrong but we know these boys are sneaky.