Using Drupal Commerce 2 in Drupal 8.8, if an unregistered user puts something in the cart (and doesn't check it out) another unregistered user sees these items in Cart block (provided by view (order)). They aren't shown to the users on the /cart page.
The reason for the behavior described above is that in Views I changed the advanced option context filter - order id - for "if there is no value for the context filter" from: "static" (the option after installation, which had the effect that no cart was shown at all) to "show all results" (with the effect described above).
I found this link https://www.drupal4u.org/howtos/how-hide-empty-shopping-cart-drupal-commerce saying the correct value would be "Current user's cart order ID", but this was not available. I'm not sure if this is a bug, if things changed for Commerce 2 or if I configured something wrong.