Really,?someone intentionally eaves a party with bottles of water they know belong to some one else, they do not have permission from that person to take the water, and you guys thing it is not stealing? What the heck is it when I take something that some one else owns?
And no, there is no party etiquette that makes this ok. At every party I have ever been to, it is commonly accepted that left-overs go to the person who brought them, until that person says he/she don’t want them or gives them away. This was clearly not done before the fact.
The OP may or may not struggle with scrupulosity, that can only be determined by her confessor. It is besides the point, a simple scenario was presented and it has a very simple answer: yes it was theft.
From the CCC
2408 The seventh commandment forbids theft, that is, usurping another’s property against the reasonable will of the owner. There is no theft if consent can be presumed or if refusal is contrary to reason and the universal destination of goods. This is the case in obvious and urgent necessity when the only way to provide for immediate, essential needs (food, shelter, clothing . . .) is to put at one’s disposal and use the property of others
How in the world could the OP have validly presumed the water was being given to them? For all the OP knew, the owner of the water had plans for them already.