Can't say I've ever had a 9 volt battery leak.
I just had an Alkaline 9V do so.
It had a bulged bottom plate, that had not had enough time to extrude it's acids, even though it still was functioning.
This cell was one that had 6 smaller cells within, typical of many 9V designs. You can WIKI 9V cell designation make up's.
There are a few different 9V designation designs to research.
I have had in Date AA Alkaline cells leak within their packaging before expiration date.
I have had many different brands leak before they had expired their output, dead, and still were passing current to power the device.
I have only experienced one exception In my days and it has been the energizer industrials sold at grainger's. They have
not leaked once in my experience, even when they had been run dead.
To this, I now I will only keep Energizer Primary Lithiums, a NiMh, or rechargeable lithium based cell(s), like Pale Blue 1.5V,
in anything I do not monitor on a constant daily basis. Alkaline cells still have their place for cost effectiveness on devices that
consume them within a day or two and are constantly monitored changed.
Whether it is a +-$200 flashlight or a +$600 occasionally used multi-meter, I know from experience not to trust any Alkaline
cells and the fact they will leak. I remove the cells and monitor them within the device to avoid any possibility and potential
of them to destroying a good working device owned. I own too many unreplaceable devices, that out wiegh the costs of an
Alkaline battery that has potential of ruining it. I even have some that advent against using alkaline cells.
Just My take
GL
.