I mean, I've not been there, but according to Congresspeople affected, they feel that it would be impossible for someone unfamiliar with the building to stumble upon their offices. Especially since even members who've worked at the capitol building often still get lost in the complex.

https://www.politico.com/news/2021/01/08/congress-democrats-...

> House Majority Whip Jim Clyburn (D-S.C.) mentioned that looters had found their way to his unmarked, third floor office and stole his iPad. He questioned how they could locate that office but not his clearly marked ceremonial office in Statuary Hall.

Let's call it a couple hundred offices in the building itself. There are hundreds of people roaming freely, checking what's behind every door. It would be strange if they didn't come across particular offices.