O and P were 2 and 3 days old, I believe.
Babies admitted to the neonatal unit often have a sepsis evaluation at admission (birth). This is based on risk factors and the delivery room history. After 36 hours or so of antibiotics, and a negative blood culture, they are considered to have "ruled out." If they show signs of infection after this point, they get a full 5 or 7 day course of antibiotics. For some reason I recall reading that the triplets had "ruled out" and were off antibiotics, but I can't find a source. Anyhow, it doesn't matter for the purpose of this comment.
Babies who, based on risk factors, did not require a sepsis evaluation can of course still become ill. Usually the first sign will be some respiratory distress: breathing fast, working hard to breathe, noisy breathing. If it is a bowel infection, there may also be abdominal distension, vomiting, and sometimes bloody stools. Sometimes there will be an event - a blue spell - an apnea/bradycardia/desat. At this point, the baby may be re-evaluated for infection using the appropriate tools and investigations (which was done). It is really quite UNusual (edit - thank you
@ColourPurple !) for the first sign of illness to be a full respiratory/cardiac arrest, especially when previous investigations were reassuring. Once again, only my opinion.