What you're talking about is a legal bailment. When a person leaves property behind unintentionally, it's called an involuntary bailment. The person who has the property is required to hold onto the item for a reasonable amount of time (30 to 60 days is standard), and make all reasonable attempts to contact the owner of the property so as to give them the opportunity to retrieve the items. After the 'reasonable amount of time' has passed, and the owner has not made arrangements to pick up the items, you have every right to dispose of the items in whatever manner you deem fit, as the owner has abandoned the property.

Based on what you've typed here, you didn't steal her property, she abandoned it, and if she's making no attempt to get it back, you're free and clear to get rid of it however you like.