I think my first instinct would be to look for more clues. Do you have a basement? Can you find where any wire(s) exit the house? Are there any external clues (conduit) which might identify where wires exit the house? Once found, could this suggest which switch controls it, if any?
Check wire at one of the fixtures. Does this confirm line voltage, or suggest LV?
Check wire at one of the fixtures. Does this confirm line voltage, or suggest LV?
Have you, in fact, checked voltage and found none?