Deina will maintain to his dying day that it's not his fault for walking in on the Senator.
Look: he's had a successful day. His proposal for simplifying the process of manumission has an easy majority, and he expects to push it through the Assembly without difficulty. He expects Valentine to appreciate this; the man usually does.
So his mind is elsewhere, and he opens the door without knocking. It's not locked. How is that his fault?