path.combine idiosyncrasies

I have this small program :

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