I defined a network that should have no flow in certain pipes (for example a dead end with no demands) and after I run my model it shows a very small flow, for example 0.0003 L/s (which may only show if additional decimal places are set to show)
What you are seeing is likely numerical noise (also known as "ghost flow"). This can happen as a result of the iterative process used by the numerical solver to converge on a balanced solution. Perfect accuracy is not always possible and the solver is able to converge within the "accuracy" calculation option, which can cause some small numerical noise. When looking at such decimal precision, this noise is usually acceptable to ignore. To attempt to force the numerical solver to balance on a more accurate solution (and reduce this noise), you can also try altering the 'Accuracy' in the Calculation Options (Analysis > Calculation Options). The smaller the Accuracy step the more trials will run during the simulation. This will enhance the accuracy of your results.
What is the purpose of the 'Accuracy' Calculation option?
Engine Compatibility Mode and related Calculation options
Flow arrows not displaying correctly per their Element Symbology setting