cocker spaniel peeing on plastic

Question
QUESTION: I have 3 cocker spaniels I have a doggie door, lately one of my cocker spaniels has been peeing on only plastic, like garbage bags, and potato bags, any thing that is plastic. Is there anything that could be causing this?

ANSWER: Is this a male who's doing this?  Where are these garbage bags that he can pee on them?

I can't think of any reason except "marking territory".
Delores

---------- FOLLOW-UP ----------

QUESTION: Yes its a male dog. he recently had surgery on his mouth because he got into something which caused his teeth to rot. and lately he has just been peeing on any thing plastic for instense I bought a bag of potatoes and he peed on it. and we were putting away christmas things at which some of the stuff goes in garbage bags and he peed on them the thing i dont understand is he has a doggie door he can go out but he insists on going in going in the house on plastic

Answer
So he's just doing this in the house?  Then it's definitely behavioral but quite honestly, the choice of plastic is quite mysterious.

How often and for how long are your dogs walked/exercised?
A lot of odd dog behavior is caused by boredom and lack of regular exercise.

If you can possibly keep anything plastic out of his way there's a good chance this will simply pass.
Delores