Leaving a bike for the first time is fine. It's new, so is just an expensive object rather than Your Bike. By the time you get attached to it, you won't worry as much about locking it up. Which is when they get you, of course.
If I'm leaving a bike for longer than it takes to visit a shop, I tend to remove all the removable that are worth anything. So lights, pump, empty luggage, bottles, etc. Probably not the emergency spare inner tube that's definitely the right size for that bike cable-tied to the frame, because that would defeat its purpose. Computers are a special case - not really worth anything to a thief without the wiring harness, but inconvenient enough to lose. So I tend to leave them attached for short stops, as prior experience suggests that the chances of dropping a loose bike computer are greater than having one stolen from a bike, but I might take it with me if I were leaving the bike locked up for a long time.