Is it normal for batteries to leak inside a sealed pack?

It can absolutely happen in the package, and it's more common than most people think. Alkaline cells can start leaking before they're ever used because of how they're built. The chemistry inside generates a small amount of hydrogen gas even in storage, and if a cell has a weak crimp seal or was slightly overfilled with electrolyte at the factory, that pressure finds the path of least resistance. You get that crusty white potassium hydroxide creep right at the positive terminal or around the seam, sometimes while the pack is still sitting on the shelf. The location you described actually matters. Batteries stacked near the entrance of a big box store are going through temprature swings all day, every time the doors open, plus hot delivery trucks and hot warehouses before that. Heat speeds up the internal gas generation, so a marginal cell that might have sat quietly for years in a climate controlled back room can start venting after a few weeks in a bad spot. It's not moisture getting in from outside. The leak comes from inside the cell. There's also just plain quality variation between production runs. Some brands are made in diffrent plants depending on the line, and a batch with sloppy seal crimps will leak at a much higher rate than a good batch, new or not.

You did the right thing walking away from those packs. I check every visible cell through the blister now too, and I try to grab from the back of the peg where stock has seen less sun and door traffic. Another option that sidesteps the whole issue is switching to low self discharge NiMH rechargeables for the stuff that eats AAs. I made that switch years ago for remotes and flashlights and haven't dealt with a leaky cell in the drawer since.

I'm really surprised that this can happen even when the batteries are totally sealed in the plastic. I always just grabbed a pack and put them in the drawer and never thougth about them untill they stopped working. If the package looks okay, but you can see a little bit of that crusty stuff, does that mean the battery is allready broken, even if the expiration date is far in the future? I guess I need a better way to check them before they run out of charge.

I ran into something similar with my own 12V deep cycle setup. I had these large sealed marine batteries, and I noticed a lot of terminal buildup creeping onto the tray even though they were stored in a closed cabinet. Turns out, a slight leak in the foundation of the garage was sending moisture right through the concrete, not just the air.

So I stopped trusting the packaging and just start checking the connection points and the casing for any hint of dampness before I even wire them up. It’s basially a constant visual check on the components you rely on. Do you guys have any cheap way to verify the health of a pack before you spend money on it?

I wonder if we should also consider the possibility of mechanical stress playing a role in these premature leaks. Sometimes, the vibration and heavy stacking that occurs during the shipping and palletizing process can induce microscopic fractures in the factory seal, allowing a small quantity of the internal caustic fluid to migrate through and deposit that characteristic white residue. It is a subtle failure mode that is entirely unrelated to the typical temperature swings you described from the store entrance.

I've been keeping a stash of cheap backup batteries for a while now, just tossing them in a plastic tote in the garage, so this is a bit of a wake up call. I always assumed if the packaging seemed okay, I was good to go. Since the leak comes from the chemistry inside, it seems like the original retail state doesn't matter that much if they've been sitting around for ages. Does the rate of this white corrosion change depending on wether I try to charge them or just use them for a flashlight?

Update: took the first reply's point about weak seals to heart and asked the store manager about it. Turns out those packs near the entrance had been sitting there thru a hot spell last month, and they pulled the whole batch and swapped in fresh stock. Grabbed a pack from the middle shelf and every cell looks clean. Guess the heat plus a shaky seal was enough.