|Feeding damage by insects can vary with the maturity of the leaf|
A. If this damage occurred to citrus leaves earlier in the season when temperatures were warm, it could because the by insects or even snails. If this is happening now, while temperatures are cold, then think of warm blooded animals such as rabbits, ground squirrels or even rats.
Ground squirrels hibernate when temperatures are cold but you may see them active when temperatures are still warm but it is winter time.
Citrus leaves will probably drop when the temperatures get cold enough and so that problem will be gone. I would not worry about it too much at this time of the year. It would be more of a problem if leaves started disappearing during warm weather when the plant is relying on the sun for its source of energy. If it's caused by warm-blooded animals, they will move on to something else.