There is a problem with "find usage". It does not find usage of signals/slots in other classes and subprojects when you are using the old-style Qt signal slot syntax like :

connect(object1, SIGNAL(someSignal()), object2, SLOT(someSlot());

But if you use the new signal slot syntax which is available in Qt 5.* like :

connect(object1, &MyClass1::someSignal, object2, &MyClass2::someSlot);

"find usage" simply finds usage of the signal/slots in other classes even in other subprojects. So it does not work with the old-style syntax of connect when it is used in other classes.

I have tested it with Qt Creator 3.3.0 both on Linux and Windows. I think it is worth to be fixed in a next release of Qt Creator. Because it seems that the old style syntax will be used for a long time by many projects and "find usage" is a frequently used utility specially for signals and slots.