What notification can come to the iPhone at exactly 18-00?

Question

iPhone

For probably a year now, every day at 18-00, a notification sound is heard. At the same time, nothing appears on the screen, the screen does not light up. Only sound.
There is nothing in the calendar, in the notes. Already clambered all the settings, but did not find the reason for this behavior.
iPhone 6S, iOS 12. It was the same on iOS 11.
Tell me where to dig. It's not that annoying, it's already stupidly interesting :)

Answer 1

maybe mail?