I'm afraid that if I will lose my laptop while turned on, somebody might take the fingerprint off the screen or somewhere else and unlock my laptop with the fingerprint sensor.
Is this a valid concern?
I am not an expert in fingerprint recreation, but a I've read quite a few articles where people have successfully hacked in devices using the fingerprint sensor via one of the following methods -
The reason it is easy is because finger print recognition like face recognition is a 'not such a great way of authentication' on the level of personal applications. The fingerprint sensor is essentially a camera that takes a close up image of the finger and its valleys. So in the way face recognition fails when you use a printed picture to pass through it, one can pass through fingerprint sensors. It works well in places where you are authenticating yourself in front of a guard, like in military or research bases. At the level of personal items they are more of a convince like me using my fingerprint sensor on my phone to not spend time unlocking my phone via a pattern or a password. Surely one can't 'read' or 'guess' my fingerprint but if an attacker is good enough, any fingerprint sensor isn't good enough. That perfectly brings me to my final point.
To quote an expert who was being interviewed at DEFCON, "A dangerous hacker is a black-belt martial artist, if he chooses to beat you up, there isn't much you can do unless you know it firsthand." Same goes for hacking, if you are really concerned and think that someone is after your data, use a good password and set up a few more steps that ensure your data is safe and not in wrong hands. If you think otherwise, use it as a convenience.