Whatsapp shows me online when I'm not in the application What reasons are there that WhatsApp shows me online, although I am neither using the application nor the phone as such. I don't know enough about WhatsApp to explain this to a friend who thinks I am lying. Could connection problems lead to a false status?
Whatsapp by default shows your status to your contact list - only. You have the option to show your status to contact list, everyone or nobody. This option can be reached by going to 


  Settings > Account > Privacy 


Based on your question/scenario, it seems that your app is not working properly. Whatsapp only shows your 'ONLINE' when you access the app or are actively on it. If you close the app - but leave your phone on, the app should not show you 'ONLINE'. I would uninstall the app and reinstall it. 

Of note, I would use an app like clean master to make sure that all files associated with the application are truly uninstalled to ensure that you have a clean re-install.