Tools like Flightradar24 and Stalkscan can be extremely useful for an online investigation. You just have to know them. The London based online investigation network Bellingcat has done some fantastic journalistic work about – just two examples – the identity of the Skripal poisoner Anatoliy Chepiga and the disappearing of the Saudi Arabian journalist Jamal Khashoggi. I participated in a three day long workshop in Bogotá, Colombia, where Giancarlo Fiorella taught tools and methodologies used by Bellingcat. Fiorella is contributor of Bellingcat and worked on the Oscar Pérez case in Venezuela. Bellingcat is holding a series of workshops in Europe and the United States in 2018 and early 2019. Checkout their website.