If there is one, there are likely more. They can multiply rapidly, and like Sunil said, if there's forage, they will likely remain.

Since you have a liner, considering the cost, I'd err on the side of caution, go Def Con 5, and make trapping the rats a priority. Seek a professional if necessary.

Colony traps deployed in runs/den entrances have worked for me in the past, and won't harm waterfowl or anything else except Mink who are hunting them and will suffer the same drowning fate.

Many men go fishing all of their lives without knowing that it is not fish they are after. ~ Henry David Thoreau

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