Heck yeah - unlocked unmanned doesn't give you right to property. Its looting, or more specifically theft.

Unlocked and unmanned is a misnomer - in most looting situations the owner will lock their store before leaving it. If the door or window has been broken, someone else has already started looting. Don't take part.

I can imagine all sorts of odd scenarios where your scenario is mitigated, such as you know the owner by name, you are a former employee at the store - maybe you could take what you immediately need and leave an accounting, and a promise to pay on demand when the owner returns. But that's seldom the scenario, we're talking B&E and theft.