Sir D's supernatural status was foreshadowed in episode 1
After being fed on by kevin, the knife and bite wound (which wasn't sealed as we can clearly see) that should be leaking blood are already fully healed and no blood is visible. We know the OP team aren't afraid to show gruesome wounds on characters, as a minute later kevin literally pulls a spear through his eye hole and it is coated in vitae. The OP team wouldn't have missed such an obvious detail, as they rendered blood from the initial knife wound, but chose to hide any and all wounds or blood from his face after being rescued from kevin.
And secondly, he got enough blood drained to be immediately delirious, yet somehow had enough presence of mind to raucously cheer about how he had spiked his blood. No normal human would be capable of thinking clearly at all let alone think to brag about the situation with lucid detail.
He was delirious after being fed on, but in a short time later (the time it took to carry the sabbat pack to the golden goose, probably not very long but it's not really shown exactly how long) he is perfectly fine again without suffering any visible symptoms of blood loss, and is capable of driving while already intoxicated. A normal human would almost certainly be fighting to stay conscious by this point considering how much blood he lost.
I feel like both of these will be obvious signs in retrospect that something supernatural is going on with D.