Remember the inflated Li-poly battery I extracted from the family tablet? I had it laying on the balcony for two weeks, where it could release dangerous gas or catch fire without much damage.
Since it looked more inflated than ever, I checked it out. The inner pressure had now reached a level where I could not press the outer film: it felt like a properly inflated bike tire!
I am not planning yet a visit to the recycling centre, so I decided to vent the Lithium cell in order to avoid an explosion. A pin easily drilled a few holes in it (in open air!!) and internal pressure was released.