How come a water bottle is squashed after you were on a mountain

At camp Derek and his friends hiked up to the forest fire lookout station at an elevation of 2444 m. On top of the mountain Derek drank the last of the water he carried in a plastic bottle. He put the lid back on the bottle tossed it in his day pack and forgot about it.
When Derek returned home to Seattle 3 days later he unpacked his stuff he found his water bottle and it was kind of squashed, how come? When he opened the bottles there was a hissing noise how come?

At higher elevations, the atmosphere has a lower pressure. The pressure of the bottle was equal to the atmosphere when he sealed the bottle on the mountain, after the desent the pressure increases crushing the bottle