Ideas
If the last of an item was taken out and there were no reorders or active orders or suggested orders to refill then it would unallocated the item from the item location but it would still keep it assigned to the device so that if it came in again in the future it could be reassigned to a new spot. This means that if you have a continuously changing stock then you do not have to keep on removing items from locations to free up space to ensure you aren't booking different items to the same locations as one another.
This is a good idea - would it be possible to have this as a selection/tick box when assigning the item to the location in the first place? So when the user assigns the item, they select "remove item from location when 0 qty is reached", which carries out the process as above? I hope this accounts for items that are on a manual reorder, e.g., someone is emailed to purchase more stock when an item hits 0 qty - this way it can ensure this items stock location is reserved.