In software engineering practice, models created for communication and documentation are often informal. This limits the applicability of powerful model-driven engineering mechanisms. Understanding the motivations and use of informal diagrams can improve modelling techniques and tools, by bringing together the benefits of both informal diagramming and modelling using modelling languages and modelling tools. In this paper, we report on an initial exploration effort to investigate the use of informal diagramming in both open-source software repositories and industrial software engineering practices. We carried out a repository mining study on open-source software repositories seeking informal diagrams and classified them according to what they represent and how they are used. Additionally, we describe industrial practices that rely to some extent on informal diagramming, as gathered through unstructured interviews with practitioners. We compare the findings from these data sources and discuss how informal diagrams are used in practice.