Step 1: Define the timeframe first: secondary PPH means bleeding after the first 24 hours, typically from retained tissue or infection.
Step 2: Retained placental bits, endometritis and a uterine polyp all fit this delayed picture, so they are genuine causes of secondary PPH.
Step 3: Placenta previa belongs to the antepartum and primary postpartum setting, when the low-lying placenta bleeds before or right at delivery, not weeks later. It is therefore the exception.
\[\boxed{\text{Placenta previa}}\]