I think it's a metaphorical expression rather than an established idiom. The idea is presumably that lying in the long grass makes him feel good, satisfied, comfortable with who he is. As Cyberpedant suggests, more context might make it clearer.
I think you caught the sense of it, Chris K. I had not notice it. This comes from The Shining Girls, by L. Beukes.
Really, I don't think that any more context would help since the sentence is quite aleatory.