Hello, Ambergotz.
I understand that you are encountering a rogue alert sound at midnight. I would certainly not be too terribly excited over a midnight message I didn't expect and hope to help.
I’ve been in a similar situation before. I kept hearing a sound every so often that I couldn't identify. It turns out I’d forgotten that I had installed an app that reminded me to drink water, and the sound was mimicking a droplet sound. As it stands, the way only sure-fire way to track down your sounds would be to work through the Notification settings of all your apps until you find something that lines up. The iPhone User Guide chapter, Change notification settings on iPhone, will take you through the how-to.
If you don't feel like working through this, you might also look into turning on Do Not Disturb during the time of the alert. You can custom tailor the time it comes on and off. Use Do Not Disturb on your iPhone, iPad, and iPod touch
I hope this helps!
Be Well.