|Fox checking out grassy area at 2:20 am, February 22nd|
They could be searching for food. According the the Collins Field Guide to Mammals, foxes are opportunistic feeders. It says that their diet includes: rodents, birds, insects (especially beetles), eggs and earthworms. We have found odd holes in the ground after visits and they are likely to be digging for worms. The guide explains that they also scavenge at bird tables and compost heaps. We have both. So there is a certain amount of food they can find while in our garden.
I'm not sure why the foxes visit. I think that, at this time of year, they might be finding a small amount of food and that is the most likely reason. Maybe they have a regular route and we happen to be on it.