It's night time here in the UK, just gone 23:00, so I can't go ask anyone now. I'm not friends with my neighbours but we're on good terms. I'll be sure to ask them in the morning and report back.

Also I never met the guy, and I live quite far away from floor 9, I'm on 4, so it would be kind of random if it was just me that got the envelope.

It does sound like a ghost, or carbon monoxide poisoning as someone else pointed out.