they talk about pram batteries here:

viewtopic.php?f=7&t=19894&p=189452#p189452

i have seen coin cell and button cell leak,

no point in worrying, life its self is just a lease.

my suggestion is when you notice your time off, take your machine apart.

change the battery or pull the battery.

if you know you are going to be storing your mac for a extended period of time,

pull the batteries

I think in some cases of exploding pram batteries, you will find they had spent some time in cold storage, and got very cold.