本来なければ usage and meaning

Question

屋敷を一回りしただけで設計図を思い描き、本来なければおかしい部屋がない事を指摘する。

What does the last sentence mean?

Just by going around the mansion once he was able to drew a blueprint, If it were not normal it points out the fact that there are no strange rooms?

本来ならば=Normally
本来なければ=Not normally ?

I am really not understanding that.
Thank you

Answer 1

You would parse it as:

[(本来、なければおかしい)部屋がない]事を指摘する。

本来なければおかしい is a relative clause modifying the 部屋.

なければおかしい部屋 means なければならない部屋 / あるはずの部屋 / あるべき部屋, "a room which should exist," "a room which should be there."

So your sentence would be like:

(lit.) He points out that [a room (which naturally should exist) doesn't exist].
He points out that [a room (which naturally there should be) isn't there].
He points out that [a room (which originally should be there) is missing].

Answer 2

It seems you got a few things mixed up. Allow me to show you how I parse it.

・なければおかしい is a clause modifying 部屋

[なければおかしい]部屋　⇒　a room [that it would be strange if it didn't exist]

・本来なければ is not the negative version of 本来ならば (It never is!)

本来 here is an adverb that modifies (or is part of?) the relative clause.

[本来なければおかしい]部屋　⇒　a room [that, normally, it would be strange if it didn't exist]

So, 本来なければおかしい部屋 is a noun phrase, acting as the subject of the clause 本来なければおかしい部屋 + が + ない (a room doesn't exist that, normally, it would be strange if it didn't exist).

Lastly, 本来なければおかしい部屋がない事を指摘する I'd translate as:

(...) points out the fact that a room is missing that naturally has to be there.

or something along those lines.

Answer 3

I agree with the O.P.'s take of the first half of the Japanese text - Just by going around the mansion once he was able to draw a blueprint, but I take the latter part of it as;

“and pointed out that there is not the room that (originally) should be there (as he envisaged in his blue print).”

本来なければおかしい means "Originally, it should be there / It’s unnatural to be lacking in it.