__DoPostBack call not being caught server side

Hi,
There's probably quite a simple and logical explination for why this is happening, but I was just wondering if theres any way around it.
I have a client JS function as below which is posting back when the user leave the page. The %DIV_NAME% is manually replaced with the name of a panel in the custom control.

    window.onunload = function()     { __doPostBack(%DIV_NAME%,"UnloadSessions");

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