I also aim to keep my greenhouse at around 3C to 8C over winter.
Theoretically this means the potted citrus are dormant and require very little water.
But, even here in England in January, the sun does come out.
It only takes half an hour of sun and the temperature passes 10C (50F).
Today, for instance, it has been just above freezing outside but the sun shone all day and the greenhouse reached 22C (72F)
That means the plants immediately use a considerable amount of water.
If they don't get any water they rapidly dry out and the leaves curl and I find a whole plant can rapidly die - particularly in the smaller pots. So it is a constant battle between leaving them cold and damp, or too dry.
I guess your Citrus hystrix needed watering!