"This password has appeared in a data leak" notice on iPhone

I had to restart my iPhone after an app froze it. For the first time ever, after the reboot I got the "this password has appeared in a data leak" notice. While I was looking it up to make sure it wasn't like one of those scam messages that appear on our computer screens periodically, it disappeared and I can't find it anywhere on the phone. I rebooted it again and this time it did not show up. I use complex unique passwords. In this situation what should be my next step?