C3PX said:
Observe:

Tarkin would have gone, “That must be where the Rebel base is! Destroy that planet!”?

The question mark (?) at the end of this quote indicates that it was a question.

He is asking, not stating. Basically he was asking you, do you actually think Tarkin would have blown up the planet the Falcon was traced to, regardless of whether or not there was evidence to indicate there was Rebel activity taking place there?

Yes, I believe that he would. Since no one in the Empire had any idea where the Rebel Base was, they would have assumed that whatever planet that the Millennium Falcon would be where the Rebel's hidden base was located.