Today I noticed that a "BD-ROM Drive" appears in My Computer.
I do not remember installing anything that would've done that.
What could've caused this?
The likely explanation would be that you installed virtual drive emulation software such as Daemon Tools, Alcohol 52%/120%, or PowerISO (or another disc-burning software that has drive-emulation capability).
In most cases depending on the version and program used, a virtual drive is added automatically after install.
BD-ROM DRIVE means Blu-Ray Disk Read Only Memory Drive.
It usually appears when you install a Virtual Drive using softwares like Daemon Tools, PowerISO, etc, and it is used to mount .ISO files and many other data formats.