I've had this appear a few times now and don't know what to think of it.
The server log shows a line like this:

"123.456.789.000:11111/12345678901234567/SomeName disconnecting: disconnect"

but without ever having a line like:

"123.456.789.000:11111/12345678901234567/SomeName joined [windows/12345678901234567]"

prior to the first. The players I've seen this with have in most cases never connected to the server or their last visit was many wipes ago. Most of those steamIDs have no VAC-bans and seem legit.

Does anyone have an idea what this could be?

Thanks in advance!

I haven't checked our connections, but we do sometimes get death notes for players that are not online at the time. I wouldn't worry about it.

you'll also see joins without them ever disconnecting.. some kicks and disconnects dont show in rcon but will show in the logs or vise-versa

@hunted5 I'm talking about the logs only. I just don't get how from a technical standpoint the server can tell you that a socket has disconnected even though it never was connected in the first place. Was it a half-assed connection attempt? If so, I still expect to see a "connect" message.

When you see that, its most likely that the user got disconnected during join session. Maybe because he was too fast on clicking on the server he wanted to join. Maybe some lag made him joining your server or who knows he had an incoming call or his parents went REEEEEEEE ya gotta eat some food or play outside BOII. 

Its nothing to be worried about.