db_owner unable to drop database - Error 615, SQL Server
I can guess you have the AutoClose option for database set to True. This is the default behavior when you create a database with Express Editions.

The mentioned error can occur exactly in this case. Actually the complete error message 615 states: "Could not find database ID %d, name '%.*ls'. The database may be offline. Wait a few minutes and try again." ... So it points that database could be closed during dropping.

So, go to DB properties, switch it to False and try again dropping it or use below script before dropping

ALTER DATABASE [MyDB] SET AUTO_CLOSE OFF 
GO


Many point out that is better to have AutoClose set to False. I found this article explaining a bit more about AutoClose: http://sqlmag.com/blog/worst-practice-allowing-autoclose-sql-server-databases

Small extension of the answer:

-- this works in standard SQL Server Editions, but NOT with Express Editions:
CREATE DATABASE [MyDB]
GO
DROP DATABASE [MyDB]
GO

-- this works in ALL SQL Server Editions
CREATE DATABASE [MyDB]
GO
ALTER DATABASE [MyDB] SET AUTO_CLOSE OFF 
GO
DROP DATABASE [MyDB]
GO