Based on the reporting in the story, they were specifically looking for what they found.
I'm not sure where in the story that is made clear and it certainly isn't clear that they had any evidence to support a belief that the building contained anything of what they actually found. I think this is all it says.... The burglary was the idea of William C. Davidon, a professor of physics at Haverford College and a fixture of antiwar protests in Philadelphia, a city that by the early 1970s had become a white-hot center of the peace movement. Mr. Davidon was frustrated that years of organized demonstrations seemed to have had little impact. I think that they were just on a fishing expedition and lucked out. As such, if they had of found nothing they would be reviled as politically minded hippie thugs. But because they got lucky and found something, they are heros? There's something wrong with that logic....