Have Steam accounts ever been silently created as part of some other registration process?

Steam accounts can not (legally) be created without knowing about it, unless someone else did it for you. Especially not with your username and some games in it. Since it has your unique name you most likely forgot about it. As far as I know the first accounts (way before HL2) didn't have to confirm their mail accounts right away, but at times of HL2 confirmation mails were sent (source: a bunch of own accounts).

That account owns HL2 games, so it has probably been created when the games were gifted to that account (or sooner). Those games require a Steam account, so there is no other way around. It is possible to create a Steam account in the process of gifting games. However, an e-mail will be sent to you when you receive a gift via e-mail. This mail can be sent to a different mail account, though.

You should still have received at least one e-mail to the address attached to that account. It's either about receiving a gifted game (account existed before) or a confirmation of your account creation, including the username.

So we can narrow down the issue:

• you forgot about it
• you lost or deleted the mail(s) from Steam (or it was flagged as spam and went into the nether)
• both of the above
• the account was created with a different mail and the mail was changed later
• someone else has access to your mail and created it on your behalf (and deleted the e-mail)