Nine Princes in Amber: Questionable Lighthouse Stay

Question

So, in Roger Zelazny's "Nine Princes in Amber", when Corwin's in the dungeon he tries to figure out how long it would be before it would be noticed, were he to go missing.

[There was an] odor of decaying flesh. I wondered, if I were to die, how long would it be before someone took notice? How many chunks of bread and bowls of slop would go uneaten before the guard thought to check within after my continued existence? ... The death odor was around for a long while. I tried to think in terms of time again, and it seemed that it persisted for over a week.

He eventually escapes by means of a magic drawing, provided by Dworkin, of the Lighthouse of Cabra. Note that in addition to depicting his destination, it was also palpably magical:

I turned quickly and considered the Lighthouse of Cabra before the match failed. Yes, the power was there. I could feel it.

He steps through the magic drawing, ends up at the lighthouse...and stays there for three months?!?

I stayed with him for three months, as I recovered my strength.

This seems impossibly foolish - he knows that his absence will likely be noticed in roughly a week, he's the most important prisoner in the dungeon, he left a magic drawing in his cell both depicting his destination and providing instantaneous transport there, and he's just going to hang out at the other end?!? Not only that, but it works! Nobody figures it out, no army of soldiers surrounds the lighthouse in the night! Is there some detail I'm missing, some note that makes this make sense, or is this as unbelievable as it seems on the surface?

(Also, I sure hope that lighthouse keeper can convince Eric that he had no idea that it was Corwin, else RIP lighthouse keeper.)

Answer 1

His escape was discovered, but not the way of it

No one thought that Corwin managed to escape via the painting. Why?

• Only Dworkin knows how to create Trumps. No one has seen him for quite a while and he is considered dead. And even if he was alive, what would he do in the Corwnin's cell?
• The Trumps are made as cards and each is a little masterpiece. The drawings on the walls are crude in comparison, made using spoon on the wall. Yes, Dworkin has signed them with his name, but that doesn't mean anything - it is like if someone would create a spray painted graffiti on the wall and sign it "Rembrandt"
• Corwin has been blinded and everyone (him included) believe that it was a permanent disability. While the drawings might suggest otherwise, there are blind painters and sculptors, so the existence of those etchings doesn't prove that he has regained his sight. Why is that important? Because blind people cannot travel through Shadows!

So whoever found the empty cell (probably indeed about week later) came to the conclusion that Corwin had outside help (there is even proof in the form of burned matches), that he left via the door and that he is hiding somewhere in Shadows. No one thought that he escaped via the Trump on the wall and that he would be still in Amber.

Even later, Corwin doesn't admit to the way he'd escaped:

"And of your escape?" he [Benedict] asked. "How did you manage that?"
"I had help, of course," I admitted, "in getting out of the cell. Once out -- Well, there are still a few passages of which Eric is unaware."
[...]
"I like to think so," he said. Then, "I understand you left the partly whittled cell door locked behind you, had set fire to your bedding, and had drawn pictures on the wall."
"Yes," I said. "Prolonged confinement does something to a man's mind. At least, it did to mine. There are long periods during which I know I was irrational.

Also, it seems that they gave up the search after a while, believing that Corwin is dead:

We looked for you. Did you know that, Corwin?" he asked. "Brand searched for you in many shadows, as did Gerard. You guessed correctly as to what Eric said after your disappearance that day. We were inclined to look farther than his word, however. We tried your Trump repeatedly, but there was no response. It must be that brain damage can block it. That is interesting. Your failure to respond to the Trump led us to believe you had died

Answer 2

he left a magic drawing

He left a drawing. Corwin knew it was magic, but there's no way a guard could know that. Only a child of Amber could have detected it and Eric didn't personally inspect the cell, as he says, "I understand you had drawn pictures on the wall", meaning he learned of it second hand.

A child of Amber would have had to inspect the cell, with good light, and presumably contemplate the image in the correct way. The second hand report of some scratching on the wall would not raise any red flags. Dworkin had been missing and presumed dead for centuries, and it's a pretty great mental leap from awareness of trumps to the idea that a crude scratching could yield the same power.