[mkr-hn]

Your lack of familiarity with the phrase and misunderstanding your own source does not actually prove anything.

Here, I'll give you a head start on doing what I suggested.

https://www.google.com/search?q=%22grow+wary%22

That sure looks like a phrase used across a wide range of contexts and unconnected fields, too wide to support any notion that it's obscure. There's even a non-English news outlet using it in its English coverage.