I guess this was expected, no matter how others - Kiel, Lispen or even Shic - planned for his unnoticed escape. Why?
1. There is no way Rood would have left behind Lidusis and Dio (he doesn't know yet our little draco is on the other side of the very door he just protected) after he went through so much trouble to secure his friends.
2. There is also no way Rood would leave behind innocents whose life was endangered - in this case the whole school of magicians, who somewhat ended up as a bait to capture BH.
3. Apart from going into the Tower for Lidusis, Rood did it also for his Master AND judging by the last panels of this chapter, he doesn't want Kiel to be blamed for the whole mess created first by Lispen summoning Oscar and then his 'lovely' chat with Shic he's having at the moment without care about poor bystanders.
And 4. Meredith already knew how Rood looks like, though never was close to his awakened form in person. I doubt he'd let Blackie go without fight either way, even if Oscar didn't appear. Since he is somehow a magician on steroids, I guess before Rood changes into Raven and regain his full power, the reason the author put all strongly possessive of BH characters in one place is to thwart the founder of the Tower, who temporarily holds upper hand.
Just imagine reaction of Lispen, Shic, Dio and perhaps Tessi and her CO. when Meredith announces for all to hear that Rood is his and now he'll never ever will let him go. Pure pandemonium, that's what will happen.
Besides, I have nagging feeling that for example, if Kiel got into grave situation (life and death etc.), I wouldn't be surprised if Rood unconsciously, even if in chains (courtesy of Meredith who seems to have some serious fetish with them), draws the energy from the previous demon king through all these tower levels and turns into Raven to beat founder's ass. How it would be possible? Well, didn't Rood already had a short chat with the chained remains of the demon king via dream/vision not so long ago? And far far away from each other in real world geographically speaking at that? Now they have only several stories separating them :) Imagine Dio's and Lispen's reaction if Rood changed visually into the very king they searched for so desperately.