Pex is based on the Daikon dynamic invariant. This document explains some results and examples in certain languages.

http://groups.csail.mit.edu/pag/daikon/download/doc/daikon.p...

Is it? I was under the impression it just uses symbolic execution rather than invariant detection.

Your are right. I mixed up dynamic symbolic execution with invariant detection. I know there is something called DySy which does do invariant detection using dynamic symbolic execution.