Why am I able to use a character that's not part of a charset (windows-1252)?

Question

You're not really using characters outside of the page and database's charset.

Because the page is windows-1252 encoded, if you enter Alt+251 into a form field and then post the data, the browser says:

"Hey this char is not apart of windows-1252 and I need to only send back data
 which is in windows-1252, so I will do the best I can and send back the 
 html character code of char &#8730;  -- oh well, I wish I could send back
 1 character, since I cannot I will send back 7."

And if you notice, this is 7 different characters which are in the windows-1252 charset.

Had the page been encoded with a multibyte charset, the browser would send back something which is considered 1 character.

So how can you query it?

 select * from tab where field like '%&#8730;%'

What you have is the html character of the square root symbol: https://www.google.com/#q=html+character+codes

Update:

Here is a very good article explaining what is happening: http://htmlpurifier.org/docs/enduser-utf8.html

 "...once you start adding characters outside of your encoding... 
 [the browser might] replace the character with a character entity reference...."

Also when you enter Alt+251 on a windows machine, it inserts the square root symbol which in Unicode it is U-221A.

Pressing Alt+251 is just a like a keyboard macro to insert Unicode it is U-221A.