path.combine idiosyncrasies

I have this small program :

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var s = Path.Combine(@"E:\andrei", UnknownVar);
Console.WriteLine(Directory.GetFiles(s).FirstOrDefault());

In what conditions , if I have an “C:\andrei” folder with a single file , the output will be the name of the file ?
And where you want to be aware of this behaviour ?
Hint : The output is : \andrei\New Text Document.txt