What container is the water in? If it is a mixing cup or glass cup, there will likely be residual fats that linger after washing. There are also various grease,s fats, and other lipids in you water pipes, faucets, and the kitchen air since particles linger around. Everything in a kitchen will start to develop a small film of grease if it isn’t cleaned constantly. It’s not harmful and you have been consuming it for literally your entire life