According to the website, they had tried to deliver my package at 5:15PM.
Now, our apartments are secure--to get into the building, you need to buzz in, however, the mailboxes are inside so the postal workers have keys. I heard neither a buzz from the outside door nor a knock on the door, and as I said, I was working in silence.
I can't get to the post office tomorrow, and more than likely I will be out at a NaNoWriMo meeting during Saturday delivery times. After three attempts, they send it back. To the UK. Normally at this point I'd just fill out the card they give you and ask them to hold it, but...
...they didn't leave a card.
Not the worst bad service in the world, but now I have to find time prior to work tomorrow to get on the phone with them and make sure they'll hold my package so I can pick it up on Saturday morning. Good job, USPS, good job.