C# XML /// Comments, where does <returns></returns> tag show up?

When you see the method in View >> Object Browser

For e.g. you will see something like this:

private int GetColumnIndex(Microsoft.Office.Interop.Excel.Worksheet worksheet, string columnName, int rowIndex, int startColumnIndex)
    Member of MyAddIn.CommandHandlers.CommandHandler

Summary:
Returns the column index with specified name and in specified row

Parameters:
worksheet: Target worksheet
columnName: Column name to find
rowIndex: Row index in which to search the column
startColumnIndex: Start index of the column

Returns:
The index of column if found, else returns 0

If you generate API documentation from the comments, it will show up in the Return value section, as seen here, right before the Remarks section.

When you choose to build an XML documentation file on compilation, then everything that is contained in the XML documentation comments gets copied over into that file. Programs like Sandcastle then can build HTML documentation akin to the MSDN from that.

It doesn't show in Visual Studio's own IntelliSense, as far as I know. But that's no excuse not to write something there :-). ReSharper's Ctrl+Q for example does show it.