Taking the Bottle
Answer: The Talmud (Bava Metzia 22a) tells about the time that three sages were offered fruits from the orchard of Mari bar Issak by his sharecropper. In contrast to his two companions the Sage Mar Zutra declined eating the fruits. Tosefot explains that the reason the others did eat was because they assumed that the sharecropper had presented fruit from his share of the crops. The other possibility – that they ate because they assumed that even if the fruits were from the share that belonged to the absent owner he surely would have consented to offer them to such honored guests – is rejected by Tosefot because this would be forbidden.
The ruling of Tosefot is based on the conclusion of the Talmud that if someone finds a lost object before the loser is aware of his loss and has expressed despair of ever regaining it, he is still obligated to return it. The fact that the loser would have despaired had he been aware of the loss does not affect the finder's responsibility. Tosefot argues that the same is true in regard to taking someone's property without his knowledge.
Other major authorities (Hagahot HaOshri and Hagahot Mordechai) concur with this ruling while the Siftei Kohen (Choshen Mishpat 358:4) challenges the comparison made by Tosefot.
In conclusion, try to avoid taking such liberty and ask your roommate for blanket permissions in advance.