There is no proof. It's just a likely scenario on a Spanish track with rabid Alonso fans, expecting their driver to destroy hamilton in revenge for canada.
The bottle wasn't there one second then it's there the next.
It wasn't the safety car driver drinking then tossing his bottle

and it wasn't the stewards. It wasn't from an airplane for sure.
It had to be one of the patrons, drunk or sober.
Maybe a disgruntled fan couldn't believe he payed to watch a "manipulated" race and tossed his bottle in protest, obviously tossing it on track in disgust of the show on stage and obviously Alonso or Massa is the last car they'd like that bottle to contact.
What we do know is that a bottle, an inanimate object, is on track and there are stands on the sides of the track filled with people, mostly Alonso fans who came to see him dominate. We also know a certain driver who is not liked much in spain escaped the SC and Alonso. These are nice little clues.