I think it is very unlikely that the "insulin labs" were sent because of suspicion that the bags were contaminated. Transient neonatal hyperinsulinemia is actually pretty common, especially in growth restricted babies or infants of diabetic mothers. The treatment is a few days of dextrose or TPN infusion and a wean off. But when a baby has a persistently high or unexpected need for dextrose, there's a particular set of labs sent to evaluate the hormones regulating blood sugar. These labs must be taken while the baby's blood sugar is low otherwise they are meaningless. They take a few days to result and then are reviewed by an endocrinologist. Depending on what they show, sometimes the baby is put on medication for a few months while the baby's insulin production normalizes. This is much more rare but not unheard of. This is probably the reason the labs were sent. They incidentally provided an unexpected result which suggested exogenous insulin administration but that is most like NOT the reason they were sent.
When doctors send labs it's only because they want to do something with the information - to make treatment decisions. In the case of the two babies, their blood sugars normalized and then there were no further episodes of hypoglycemia. If there is no more hypoglycemia then there is no further reason to consult the endocrinologist or to make a decision about medication. Also those bags of fluid were long gone. So, by the time those results came, 1) there would have been no way to re-take them, and 2) it's possible the results were dismissed as specious or clinically irrelevant, since the babies had apparently recovered with no harm done. It would have been best if there had been any kind of follow up at all, but there was also incomplete information, hardly enough to even enter as a Datix. I actually don't think, at the time, that it was as big of a failing as it was made out to be now.
Again, just my opinion only!
edit- At least where I work, if we get a result that is crazy unexpected, our docs usually want to repeat the test before making treatment decisions. Those results may have only become meaningful in hindsight.