Breakpoint on hidden field change

Hello,
I have a hidden field (string) in my asp.net page. I cannot find the place in the code where it changes to empty string "". So, i put a breakpoint into Page_Load first bracket {, and mouse right-click, chose "Condition", MyHiddenField.Value, "Has changed" OptionButton. So, the breakpoint stops when the MyHiddenField.Value changes, but it stops on Page_Load. I want to know the place where it changed, the code MyHiddenField changed.
I also tried to use "Find tool" (Ctrl+F)  - where i have MyHiddenField in the code, but i couldn't find it where it equals to an empty string "".
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