Why does BinaryWriter prepend gibberish to the start of a stream? How do you avoid it?

They are not byte-order marks but a length-prefix, according to MSDN:

public virtual void Write(string value);

Writes a length-prefixed string to [the] stream

And you will need that length-prefix if you ever want to read the string back from that point. See BinaryReader.ReadString().

Additional

Since it seems you actually want a File-Header checker

1. Is it a problem? You read the length-prefix back so as a type-check on the File it works OK

2. You can convert the string to a byte[] array, probably using Encoding.ASCII. But hen you have to either use a fixed (implied) length or... prefix it yourself. After reading the byte[] you can convert it to a string again.

3. If you had a lot of text to write you could even attach a TextWriter to the same stream. But be careful, the Writers want to close their streams. I wouldn't advice this in general, but it is good to know. Here too you will have to mark a Point where the other reader can take over (fixed header works OK).

That's because a BinaryWriter is writing the binary representation of the string, including the length of the string. If you were to write straight data (e.g. byte[], etc.) it won't include that length.

byte[] text = System.Text.Encoding.Unicode.GetBytes("test");
FileStream fs = new FileStream("C:\\test.txt", FileMode.Create);
BinaryWriter writer = new BinaryWriter(fs);
writer.Write(text);
writer.Close();

You'll notice that it doesn't include the length. If you're going to be writing textual data using the binary writer, you'll need to convert it first.

The byte at the start is the length of the string, it's written out as a variable-length integer.

If the string is 127 characters or less, the length will be stored as one byte. When the string hits 128 characters, the length is written out as 2, and it will move to 3 and 4 at some lengths as well.

The problem here is that you're using BinaryWriter, which writes out data that BinaryReader can read back in later. If you wish to write out in a custom format of your own, you must either drop writing strings like that, or drop using BinaryWriter altogether.