Implicit async custom validators

[Barry Kaplan]

After doing a big upgrade I'm getting the message 

Implicit async custom validators (custom validators that take 2 arguments) are deprecated in mongoose >= 4.9.0. See http://mongoosejs.com/docs/validation.html#async-custom-validators for more info.

But I can find no uses of async validators. Indeed I only have a single use of a validator. And even when I comment that out I still get the message.

I have searched the mongoose for the strings in the message cannot find it. 

Any clues on how to track this down?