Default Schema allows dbo table selection without using schema prefix
You don't need the prefix when some object is in your default schema because SQL Server checks for a non-schema-prefixed object in your default schema first. It will then check the dbo schema, so you can actually get away with this laziness in two different ways. 

From the documentation:


  When database objects are referenced by using a one-part name, SQL Server first looks in the user's default schema. If the object is not found there, SQL Server looks next in the dbo schema. If the object is not in the dbo schema, an error is returned.


They say "one-part name" but this also applies when you reference an object with a pattern like:

[server].[database]..[name]


or

[database]..[name]


This is nothing but a confirmation that you should just always specify schema name. Always.


Bad habits to kick : avoiding the schema prefix