Well, whatever this key was, it was for something important. An intricate design, likely made to prevent lock picking from happening easily. Not to mention the material - it likely cost a bit of money to create well. Well, first things first, she'd test the key on the large set of double doors. If it fit, well, then why not? She'd go in and take a look. She'd need to be careful of course - lock the doors behind her and then see if there was anything of value beyond those doors. If it didn't, she'd casually pocket the key and then walk away. She'd spend the rest of the night seeing if she could find a lock to fit the key.