Here's the deal, I always tell the customer that it may need adjustment once it gets dark out, since the sensor in test mode is not 100% accurate. I show them how to move the head and also how to turn the sensitivity knob
Problem is, you told the customer you would come out and " twist it for them "
If you charge them for it, they will probably pay but don't expect any repeat business or any refferals. What I would do to cover my self is call them up and say that you won't be in the area for at least a week but that they can adjust the motion head just by turning it and that it is best done at night. No tools are even needed. If you luck out they will say that they will adjust it.
It also depends on the customer, if its a little old lady I wouldn't tell her to break out the 6 footer and start adjusting. I would just go do it