Just FYI Ellie, there's usually a date on batteries these days, it's a "Use By" date but it doesn't mean you have to exhaust the battery by then, it literally means use it by that date. After that, without being used, the self-discharge rate becomes incredibly high and the battery goes dead fairly quickly.
However, using it before that time helps reverse the chemical reaction that causes the self-discharge. Seems like you were just unlucky with that set of batteries but lucky with another new set - glad you've got it sorted!
PS - if you would like to add a flashing LED that comes on and turns off automatically with no external activation etc, give me a shout - it's purely a deterrent but is cheap and usually effective.