Looks like there's more to this story. Still not sure how this equates to "terrorism" though.
First, as we learned from The Guardian itself, Miranda was not just traveling or on vacation -- he was specifically visiting Laura Poitras, a U.S. film-maker based in Berlin who had also received many of the stolen documents from Snowden:
While in Berlin, Miranda had visited Laura Poitras, the US film-maker who has also been working on the Snowden files with Greenwald and the Guardian.
And then, after finding out that Poitras was working with the same stolen, classified materials as Greenwald, we subsequently learned from the New York Times that Poitras is in the practice of using others to transport and hold her sensitive materials for her:
After being detained repeatedly, Poitras began taking steps to protect her data, asking a traveling companion to carry her laptop, leaving her notebooks overseas with friends or in safe deposit boxes. She would wipe her computers and cellphones clean so that there would be nothing for the authorities to see. Or she encrypted her data, so that law enforcement could not read any files they might get hold of. These security preparations could take a day or more before her travels.
So, already the story sounds much different than Greenwald's initial claims. And now we learn, again from the NYT, as now confirmed by Greennwald himself, that Greenwald and Poitras were, in fact, using Miranda as a mule to transport the stolen classified documents:
Mr. Miranda was in Berlin to deliver documents related to Mr. Greenwald’s investigation into government surveillance to Ms. Poitras, Mr. Greenwald said. Ms. Poitras, in turn, gave Mr. Miranda different documents to pass to Mr. Greenwald. Those documents, which were stored on encrypted thumb drives, were confiscated by airport security, Mr. Greenwald said. All of the documents came from the trove of materials provided to the two journalists by Mr. Snowden.