I have seen PP3 leakage, maybe two or three times in my life (52 now). It's vanishingly rare compared to AA/AAA, but it can happen.
i've had a us$350 fluke multimeter destroyed by a leaking PP3 (9v) battery. i found this quite upsetting. also several other multimeters (non-fluke) badly damaged by leaking AA cells, not to mention various vintage calculators.
note that a non-rechargable PP3 battery may contain one of several different cell configurations internally, including:
1. 6x AAAA (cylinderical) alkaline,
2. 6x AAAA (cylinderical) zinc-carbon,
3. a stack of rectangular zinc-carbon cells, usually wax coated,
4. a stack of rectangular alkaline cells, usually plastic encased.
each configuration will have different 'leak characteristics'. when fluke started using PP3 batteries back in the 1980's, by far the most common variety was composed of rectangular zinc-carbon cells with a wax coating. added to this is anecdotal evidence that modern alkaline cells in batches randomly have a propensity to leak, where one brand may have no problems one year, but a high rate of leakage in another year.
i believe that the reality is
many alkaline (in particular) cells and batteries manufactured these days are of poor and/or inconsistent quality and should not be trusted in anything other than 'disposable' products. ie: torches, smoke alarms, children's toys.
my apologies to the original poster for going so far off topic.
cheers,
rob :-)