And any personal work you might have done using your work laptop now has a legal opening for your employer to claim ownership to, since you developed it using company assets.
I built and sold a company on "Work machine" since I've been employed here in the last 7 years. This setup only works because of the company and the culture. I can sympathize that it wouldn't work for larger employers.
Definitely the case in the US. I don't know about Europe. It can feel silly thinking about this in our industry with laptop usage. However, for the general legal reasoning consider something like using a company server farm or a chemistry lab for research on your side project. The difference is only in scale.