Is it possible for sepsis to go undetected?

Doubtful. Sepsis is a severe, life-threatening illness that almost universally ends a person up in the hospital. Typically they are very ill, with high fevers, confusion, low blood pressure, etc.
Not for long. Sepsis may be misidentified at first, but it quickly becomes apparent after the first couple hours. There are no silent septic episodes - they all cause significant symptoms.