There are all sorts of crimes you can commit because there is nobody watching you 24/7 to see that you don't, and only a chance of being caught if you lie, eg not paying your taxes honestly, or not always driving at or below the speed limit.
Having said which you can write what you like in your log book - it only becomes a crime if you then use the untrue information for some purpose for which true information is required, such as a licence or rating issue or a job application.