The thing is that you don't necessarily see it anywhere (other than in the code). Basically somewhere deep in the standard code there is a code that triggers the event. If you know your ABAP and can do some debugging you can find the code.
Sometimes you can have an effect whether the standard events are raised or not. SAP gives you an options for this. In these cases, you can normally configure it somewhere in the IMG. For example, if I remember correctly there is a setting in IMG for parked documents, and when you set the on, the standard event FIPP.PARKED (might remember the name of the event wrong) gets triggered.
Do you actually have some problem with the event(s), or are you just curious?
Regards,
Karri