Using a possessive without an object

You can use a possessive without an object if you know what the object is.

You: Whose jacket is this?
Your friend: It's Bob's.

You can also use a possessive pronoun without an object if you know who you're talking about.

You: Whose guacamole do you prefer, mine or Jessica's?
Your husband: Of course, I prefer yours.

Finally, when you're talking about a person's home, you don't have to use the object if you're using the person's name or title.

You: What are you doing tonight?
Mary: I'm going over to Bob's.

So when you read or hear things like "I always spent summer at my grandma's" or "the party will be at Mary's," you know that these possessives refer to homes.