Hi, I will teach my issue with the examples : 
We created a php project i netbeans. And we want to search a word in project. The search window was opened. We select specific folder in the project folder in search scope, we  searched a word , then closed project.We created another new project. Again, we want to search a word. oh my god! When we open search window  , search scope is old. it is previvous project's scope. 

So, when we create project, old search directory must be cleaned. We may search wrong unless we care !