Learn English – what does “is off by” mean in this context

This is/was clearly X.

Simply means that the circumstances make it apparent to the observer that X is true.

Without knowing the full context of the statement, I can only speculate, but lets imagine a passage such as:

She woke up at 10:30am, took a long shower, then made coffee, and sat on the sofa to read a magazine. This was clearly her day off.

One can see from the circumstances that the woman is in no hurry to get to work, or other commitments so this was clearly her day off.

The key is the word clearly, which has several synonyms in this context, such as obviously or apparently...

This was obviously her day off.
This was apparently her day off.

Or even some similar phrases:

This seemed to be her day off.
It looked like this was her day off.

It means something like 'isolated', in the sense of being closed so that nobody can go in. It's an unusual way of expressing this meaning, so I'm not surprised you couldn't find it. I would say it's the result of combining two more standard phrases, in the way that people often do.

The first of the two phrases I'm thinking of is

The location should be locked

which means that access should be forcibly prevented by a mechanical lock on the door. When a location is locked it's not necessarily obvious that it's locked unless you try to get in.

The second phrase that is combined with the first is

The location should be closed off

Which means that it should be designated as inaccessible by some obvious physical means like a barrier or signage, but not necessarily locked. If you combine the two you get

The location should be locked off

So, why did the writer write it in this unusual way? I have some ideas, but I don't know for sure.

• This could be a colloquial usage in an English speaking area that I'm not familiar with.
• The writer might not be confident that either phrase really expresses what they're trying to say, so they use both together.
• The writer is being inventive. Americans in particular like to invent words if they think there's not another word to express what they want to say. There usually is a way to say it but making up new uses of language is seen as being clever.
• The writer wanted it both locked and clearly designated as inaccessible, so they combined the two ideas, but in a way that's not very clear.
• They were thinking it would be one or the other but they didn't know which at the time they wrote the sentence, again failing to be clear about what they were thinking
• The writer wanted to emphasize the importance of the location being inaccessible and instead of saying so plainly, he or she combined the two redundant ideas to double their impact.

The fact that the writer included a third redundant phrase, 'and secured', adds to my suspicion that they weren't confident of being able to express their meaning successfully. They actually used three different ideas that mean roughly the same thing: 'locked', 'closed off' and 'secured'. Any one of them would have been sufficient to express the meaning by itself.