I agree with Jamie there is a very strong chance that something outside is triggering a territorial marking behavior inside. I had a female that horrifically got up set at even the site of squirrels and would mark & howl at the site of them.
One product that did work well at discouraging visitors on our property was called the ScareCrow. Which is a motion activated sprinkler. Of course it may not be an option now with winter and all depending on where you are from. But it work very well with the squirrels and visiting neighborhood cats here in the warmer weather.

Good Luck!
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Cheri,Shelby,Lily & Lucy

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