I'm not going to look after my colleague's (also boss's gf) bags while she's away
Why won't you look after her bags?
I don't want to take care of her bags, I'm sorry.
This colleague of yours is not that close to you, so you're not expected to go out of your way to watch their stuff. This is especially true when they haven't asked you. They may not even want you to watch their things out of fear for their privacy.
I think I will keep their bags in here and hopefully they'll think their bags are missing.
That doesn't sound like a good idea. Just don't mess with them since they didn't ask you.