I wouldn't.
You bring up something interesting: if I open an bag and see a pic that is child pornography, and then as a TSO I continue to search the bag for more child pornography, I have violated the administrative search directives TSA was task to do, and the case against the person would most likely be thrown out of court.
At the same time, if conducting a search and I open a bag and see a picture described above, and at that point I look no further but summon a LEO who continues to search, then that would hold up in court.
The same holds true for ANYTHING I suspect of being illegal. I have to stop my search when I either find something I clearly know is illegal or what I BELIEVE is illegal.
As far as a nude child running through a sprinkler, I think that is a common enough picture (or something similar) that many people have of their children that most would NOT believe it was pornographic (ugh...just realized if a TSO did stumble on a pic like that and didn't report it and it turned out later the child was a victim, there would be a thread here bashing that TSO and TSA for not turning that person over to the LEOs and letting a sexual predator escape, assuming he was later caught. You know that it would happen, don't tell me there wouldn't be a thread here like that!
). So if a TSO did nit think they stumbled upon anything illegal they would have no reason to get a LEO.
As for flour, I guess it depends on if there is any doubt in the TSOs mind. One TSO might be suspicious and summon a LEO, who would take over, and another TSO might not think they witness any illegal activity, so why summon a LEO? Is it possible drugs get through this way? You betcha; do I care if drugs can get through our checkpoints? Not really. And for the record I do NOT support drug use; I hope and wish people know better. Except for advil. Those things work wonders on my oh too common head-aches!
As for a LEO searching a bag, once they take over, several things. They are not under the constraints that TSA is under. They can specifically look for narcotics, cash, fake/stolen IDs. And it's do so as an adminstrative search; the passenger at the point, our courts have rule, have given their conscent by submitting their bag for inspection and they themselves progressing into the checkpoint of their own will.
I think it would be a weak argument to claim that the passenger submitted themselves to being searched by TSA but not the police. I do not think any court would buy that.
Also, it could easily be argued the police at the point they take over the search have reasonable suspicion, as they were told by a federal officer (sceener :P ) that they believed they were witness to something illegal. I very much think our courts would agree with that.
Eh, but these are just my opinion. Yours may differ.