I am calling BS on this. Not only have I recently returned a phone that was rooted and not been charged, but when you wipe everything (and I mean EVERYTHING), there can be no trace of what was running. My guess is (like others have stated), if the person was charged, they didn't remove everything. They probably just reset everything thinking that will take care of it. If this were true, we could be hearing about this all the time...just my $.02
Also, here is something to think of...my last phone I got from VZW was rooted when I got it! I laughed and to have some fun, I called VZW support and mentioned this to them. At first I was told this is impossible, as they fully factory reset everything. They do a factory reset and wipe the cache is my guess (at least that was all they did on my phone). Just throwing that out in case anyone is scared over this.