It's funny you mention that, I actually had logged in this AM to use those funds to add to a holding and realized they weren't there anymore. It isn't a margin account, but I'd be willing to bet they would have liquidated something to cover the deficit if I had done that. It is in my 401k. Oddly enough they removed the entire transaction from the history, there is no In and Out anymore it is just completely gone.
I also am curious what they would have done when I initially bought the position about the accrued interest. I had to call to buy the bond and I asked what the accrued interest was before placing the order. The broker said there was none, he sounded pretty unsure and I was pretty confident he was wrong so I asked again and he said nope not showing anything accrued at all...they definitely took the money for the accrued interest when it posted. I was a bit annoyed, but like I said I was pretty confident he was wrong so expected it.
Last year with EXXI Etrade did that as well on taking the interest back out. It took a few days before they took it back out surprisingly. I'm guessing they must have some automated method of depositing the scheduled payments and then if something changes they just pull it back out.