Yes, I think that's a plausible reason they're not getting any leads out of these boys -- because the boys just might not know what happened to her after they left.
An investigator who came on right after Greta seemed to be thinking in that direction too. He said the fact that investigators apparently weren't getting any helpful info out of any of the five boys arrested, after all this time, might be that there was little to get out of them [paraphrased].
I'm going with Greta's instincts.