Can someone have a kidney infection from untreated uti and not know it until they just drop dead?

Anything is possible: Untreated uti's can turn into "pylonephritis" and that could lead to dehydration and sepsis. Sepsis could lead to multi-organ shut down, heart failure and ultimately death. We see this happen in the nursing home when residents are neglected or don't complain for weeks, until they get a fever or abdominal pain.

VERY unlikely: Kideny infection causes fevers and significant pain - particularly in someone who is not elderly. If you think you have a UTI or sexually transmitted disease, see a health care provider.