He's definitely not the librarian, as it took days of watching and planning to find a way into the library.
The praetorian guards were searching for Titus, which was a very common name in roman empire - this means they're searching for a roman. Romans patricians had the good jobs, like librarian or architect. For a farmer or a healer to be a roman, that was pretty rare. As it was not a librarian, the smart guards might have deduced that the only one left is the architect, which also knows how to build things.