The Madness Continues

I have a script which identifies MDF files which do not appear in my list of databases.

The purpose of this is to find databases, usually on development boxes, which have been detached and forgotten, but are taking up space on the disk.  A detached database is of course invisible, unless you go poking around looking for it.

What I usually do is:
i)  attach them again
ii) take a final backup just in case they actually are needed
     (not very likely, but sod's law says if you don't do this, they will suddenly become critical)
iii)  drop the thing

But today, I failed at the first hurdle:

Create file encountered operating system error 32

The process cannot access the file because it is being used by another process

SQL Server error 5123 tells me it's a problem attaching the database

Huh?  It's a detached database file, isn't it?  

What could possibly be using the detached file?

Some utter maniac renamed the .ldf file as .mdf

name	fileid	filename	filegroup	size	maxsize	growth	usage
GenDraft	1	F:\User Databases\GenDraft_20180925.mdf	PRIMARY	40960 KB	Unlimited	8192 KB	data only
GenDraft_log	2	G:\Logs\GenDraft_20180925.mdf	NULL	12352 KB	2147483648 KB	4096 KB	log only