After picking a different inn than the one you're staying in, you find an acolyte visiting the call of nature.
About five minutes later, you've knocked the acolyte out, taken all his stuff, including clothing, weapons, etcetera, and left him knocked out in someone's back garden. That should take a bit. There's no front door key, but there are three keys. "Laboratory #3", "Floor 2 Ritual Room" and an elevator key.
A quick disguise later, and you're in the front door. They're busy all night, but if you act like you know where you're going, things will go fine. The elevator isn't monitored, so you take a good five minutes making a master key for it, letting you check out all the floors. There are a couple conference rooms with trophies and examples of past work, but no Sword Valterre. There is one depiction of Spar and Dorren, but it's not current. You know Spar currently wears Adamantium Plate of some kind, and this depiction of her doesn't have that, and to make matters more obvious, there is no scabbard. No scabbard means no Valterre.
There is a records room on floor two, but it's recent stuff. Valterre was quite some time ago. You want the fourth-floor Archives. The desk there is manned, and you figure the archives room takes up a good quarter of the floor, so it'll take quite some time to search.
How do you want to handle this? You could try to hide and sneak in after the archivist has checked out, but that might take quite some time to find what you want. You could maybe work something on this person, but the more face-to-face encounters you have the more likely someone notices you.