I have permission on ground near Stansted. I did notice that this year there were fewer muntjac around. I don't know if it is coincidence or not, but they have recently had a big problem with hare coursers on the ground. Piles of dead hare have been found in fields, but there has been no evidence to suggest they were after any deer.
Can't help thinking it might be contributing to the lack of sightings, either through dispersal or them just sitting very tight (I bumped one on Thursday 6ft from my feet, it then did a belly flop into a stream before hightailing it - first time I've seen a swimming munty). Nevertheless, there may be some other explaination... especially if it is happening in other areas.