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?
I haven't checked the bulbs yet, but my first order of business was to check each light fixture to see if there was any power on it with a volt meter. Problem is, I don't know which switch in the house or garage turns on the outdoor lighting which is in the front and back.
Have you, in fact, checked voltage and found none?