I have a file 4 bytes long called data.txt with following content (four 8 bit values in hex format):
31 09 31 09
When I open this file in notepad (as well as Ultra Edit Studio) it shows single square instead of expected : 1 <tab> 1 <tab>
By what logic leading 31 09 gets interpreted as an indication of non latin1 charset?
The short answer is that Notepad guesses the encoding, and often guesses wrong. There are numerous discussions on this; Google for notepad character encoding for example. It is difficult to say in each specific case why the guess is wrong. Moreover, the situation varies by Windows version and may depend on other things as well. In my environment (Win 7, Finnish version) the specific problem does not appear: a file with those four bytes opens normally in Notepad.
If you know the encoding of a file, launch Notepad, issue the Open command, and select the encoding from the dropdown menu. If Notepad does not support the encoding, use another editor.