A number of years ago while working at an air bag inflator manufacturing facility, I went through training for the leak detection systems in use. The inflators were gas filled to about 7,500 PSI. The gas charge included helium to serve as the detection element for leaks. What I learned in the training was that EVERYTHING leaks. Including multi inch thick glass. The only thing that changes is the leak rate. And as noted above with the atom / particle size compared to the porosity of the container. And note, there is no such thing as zero porosity. It was a perspective light bulb for me when I learned this.