I'm going to hide my girlfriend's hoodie
Why would you want to hide her hoodie?
I do it to keep the hidden items for my own use.
You shouldn't be taking things that don't belong to you without permission.  Your girlfriend might worry that she has lost her sweatshirt, and get stressed out about it.  I'm sure she'd let you borrow it if you just asked.
It's better this way.
I can understand wanting to use your girlfriends things but you should ask. Your girlfriend will be upset that you keep taking her things and it might cause problems in your relationship.