Muntjac are widely known to be spreading across the country. Unlike the other deer species in the country, they are prolific breeders and can have multiple young every year. Not only does this lead to a natural expansion in search of food and habitat, but the deer are largely territorial and stay in family units. This means that shortly before the does drop their new fawns, the young from the previous birth are chased out of said group and look to move on. It could be that the deer you saw was a youngster looking to set up its own territory. That being said deer of any species will move from one area to another if they are overly pressured or if the food and cover levels for them get too low. Muntjac have also been known to spread through illegal releases, the general thinking being this is to provide unlawful sport to unscrupulous characters. If you’ve seen one, keep your eyes open as there’s likely to be more!