Labels is the first part of the 7th episode of season 2, which first aired on August 27, 1997. In this episode, Dexter is fed up with Dee Dee taking his things. He invents a label-making gun to counter this issue. Dee Dee later makes her own label gun, and chaos ensues.
With the toilet labeled by Dee Dee, Dexter apparently went to the bathroom on the upstairs carpet. Note that Mom mentions the carpet being wet.
The members of Dexter's family seem to have different points of view when it comes to labels. When Dexter or Dee Dee see someone's name on an object, they think it belongs to that person. When Mom or Dad see someone's name on an object, they think that is the name of that object.