This is the kind of statement that shows how some scientists have difficulty admitting that they don't know all there is to know about science.
It's incredibly arrogant to say that something shouldn't be there when it now has shown to be there. But, he couldn't just say, "We don't know why it's there."
I think you are mistaking a figure of speech for a dogmatic assertion. We all use figures of speech in common everyday language. “I love meatballs,” or “I’m sweating my ass off.” There are literally thousands of examples. The scientist saying “that shouldn’t be there” is using a figure of speech to say it is something unexpected, something new to add to his knowledge and to be explored. Not arrogance, but wonder.
I'm all for exploring space and funding missions, but not when its central desire is to find life. There isn't any, and its a futile exercise that wastes millions of taxpayer dollars.