>You can check which connections are still opened to DB
>
>https://social.technet.microsoft.com/wiki/contents/articles/22702.sql-query-to-check-number-of-connections-on-database.aspx
>
>BTW, I often run into a situation when I leave the query window opened and next morning I'm unable to re-connect (all options are greyed out) :(

This is where I was at yesterday. If the single user wouldn't have worked, I would have checked that out.

Now that we talk about it, this situation seems to be like that since I installed Power BI Report Server. That server seems to keep a live connection to the database even if I am not using it. So, basically, as soon as I restart the service, it would reopen its connection. That might explain the behavior I faced.