Wipe it away where you can find it. Clean the pipes going away from the reservoir. Check the area around the reservoir.
Sit in the car start the engine and have someone steering. Watch the reservoir, pipes, connections, especially to the pump and power steering box. Use a torch.
Nothing? Drive and check then again.
Then check if oil returns. Remember, if it was spilled it could return without a leak, so try to clean it as good as possible.
However, if it was just spilled (when could that had happen?) and you have driven many miles and parked there several times, you should have seen the spots earlier.
AWo