Recently I've logged into my Webmaster Tools and noticed that I still got verifications on some of the sites that I do not own anymore, for like several years already. It suddenly brought multitude of questions to my mind, for example there were sites that I did together with some ex-partners, that I do not communicate with anymore, because of personal reasons. Does this mean that they still own some rights to my sites from Googles point of view? Shouldn't verification auto expire at some point in time?
Does this mean that they still own some rights to my sites from Googles point of view?
No, the authorization is one way (i.e. you having access to get webmaster tools data from the following sites) if it is granted. So you having them does not give them any rights to your account nor any other sites that you might have connected to Webmaster tools.
Does verification that owner of domain undergo on Google ever expire?
No it does not expire but it can be manually removed. If you click on "Manage site" there is an option at the end "Delete site".