Being this an ex-leasing computer, this should be an OEM copy of Win10. It should be legal to sell a PC and transfer the license, right?
I have looked for windows stickers on the body of the notebook and I found none, but this being an ex-corporate computer, maybe it didn't even come with one, unlike customer products.
That depends ...
Corporate machines are usually not sold with licenses, the licenses come from a volume licensing deal the company has. The PC comes with an unactivated version of Windows that the corporate IT usually promptly wipes and replaces with their standard disk image.
Another possibility is that the license associated with that machine is activated elsewhere (e.g. on another PC of that company) so you, logically, can't activate it again, regardless of the fact that the PC has been sold. Common problem is decommissioning the old machine the license has been associated and with and forgetting to deactivate it on Microsoft's website.
Win10 also has such a thing as an "electronic license" - if you have upgraded from XP/Win7, then your license likely got converted to this, associated to your Microsoft account - and then it can be reactivated only 3 times or so.
Then there are also the OEM licenses that are tied to the hw they were sold with and will cause problems activating them on a different PC. There is a provision for a PC upgrade/significant hw changes but also only limited amount of times.
And sometimes even legit licenses without any restrictionssimply won't activate for completely arbitrary reasons or bugs in the Windows activation process. I had to even call MS support once for this and their support guy was completely stumped why it won't activate. Legit retail boxed (not OEM) license, with all the paperwork in order - and it still refused to activate it.
It is a mess that MS has no motivation to fix because it only hurts retail customers, not businesses - and there the goal is to push them to buy another license, not to keep upgrading/transferring their old one.