We recently removed the requirement that users be logged in to download models. This is why you'll sometimes see 'someone has downloaded your model' rather than a username. You will still be able to see the username of those who have an account.
Designs cannot be added to a collection without having signed up for the site. You will be able to see the username of anyone that has added your design to a collection. If this isn't the case, feel free to reach out by emailing 'email@example.com' so we can get everything sorted out.