Change in behavior

I have 2 dogs - Foffy, male, 4 years old and Tiffany, female, 4 years old. Foffy was trainned in order to use only the same newspaper-covered corner as his toilet. Without any apparent reason, Foffy started to use a few other places he choosed, like the furniture feet. Tiffany never uses the newspaper-covered corner if it has already been used by Foffy. These behaviors usually happens at night, when everyoune in the house is sleeping or when there is no one in the house. By the way, we live in an apartment, and the dogs are used to be free around the house but when everyone is away - we close the bedroom & bathroom doors to avoid problems. Does anyone had a similar problem or any suggestion to correct that bad habit??
Raquel Velloso Blanco

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