These emerged from a hidden corner of a basement shelf, where they’ve been sitting undisturbed for far too long:
I’ve known for a while that the PETE plastic used for nearly all bottles isn’t completely waterproof, but never had occasion to measure the results.
The laser-etched date code on the bottles says they “expired” in late August 2012, so, assuming one year of shelf life, they’ve been quietly evaporating for five years.
Sampling a few bottles shows a nearly uniform weight of 459 g. A drained bottles weighs 13 g, so let’s say the bottles now contain 445 g of water. They should start out with 500 g, although I’d be mildly surprised if it wasn’t a bit over that to prevent some dork from complaining about getting only 498 g.
Rounding in all the right directions, losing 60 g during five years works out to a tidy 1 g/month in a basement room at 60% RH.
The surface area of those wonderfully convoluted bottles might be 300 cm², so they lose 3 mg/cm²·month.
They’re near enough to 0.10 mm thick, which I’m sure is a compromise between reducing weight (and, thus, plastic cost) and incurring messy failures during normal handling. The evaporation rate surely varies as an inverse exponential of thickness, but I’m not going there.
I’m certain water bottlers know those numbers to several decimal places and can plot them versus all the interesting variables.
Memo to Self: don’t lose track of the water bottles!
The Smell of Molten Projects in the Morning 
The 60% humidity would be a factor. Outside vapor pressure would reduce the evaporation rate. – But that all assumes constant pressure. A full bottle might be under slightly positive pressure. Higher evaporation rate in the beginning. The walls getting sucked in would indicate the opposite effect. And it’s seasonal. I wonder what effect the moon and the Mayan calendar have …
Dunno about the overpressure, but I’m certain the Mayans have something to do with it …