I bought one or two ammo cans when I ran across some at a soso price, so lots of stuff went in ammo cans.
Depending on what you put in there, it will contain what leaks.
I don't really expect an issue with a leaking bic making the ammo can do much more than smell funny when you open it, but probably would not want to open one up that had been closed up for years and years while sitting right next to a camp fire.
The little yellow bottles of lighter fluid, forget the name, leak over time. Some smell from them but not a big deal.
We all know a filled zippo slowly lets fuel evaporate out as well.
My issue with flints has only been when they were used, I take apart old disposable lighters for the flints when bored. I don't really go crazy storing them but I guess since I have a vacumn sealer I could see vacumn sealing them next time around.