How do you decipher when and how to use 'off-guard' or 'off guard'?

Example sentences

“I wanted to find it before my opponents did,” he clarified. “So, if anything was brought up during one of the many townhall debates we had, I was ready to respond and defend myself instead of being caught off guard.”

In hindsight, this was a fatal mistake, as Superior Holt caught us off guard and alone in the vestibule as we were attempting to leave.

The lack of context to his question caught me off guard.

Those are the three times in my novel where I use the phrase "off guard". None of them refer to "guard duty" or any other job. They are merely a synonym for "by surprised" - but I'd prefer not to use that phrase.