if there are hinds you can almost guarantee there will be stags, so who knows what may happen if they are left alone for a bit...
Actually that is a very good point - stags will tend to move about and pop up here and there. Seeing a sika stag means nothing, even seeing a good sized group of them, as they could be 30 miles away the next day. However if you are seeing hinds then you almost certainly have a population which MAY be resident. It has also been found that stags often move into an area before the arrival of hinds, the slight problem being that it can be a very many years before the hinds follow.
The catch with this is that sika also migrate. Sika hinds tend to live in a very small area, usually a few acres, but they will sometimes migrate between a summer area and a winter one, this is well documented and I've also got personal experience of it. The actual departure for migration tends to be rather piecemeal and seems to happen any time between the end of October and the end of January usually with small numbers of deer leaving at random times during this period but with the potential for them all to leave at once. However, they all tend to move back in a relatively short time span, in my case usually about the middle week in March, and they appear to do this as a group unlike the normally piecemeal departure.
So your westerly hind might have been on its summer ground, or winter ground, or moving between the two.
As jamross65 has said they are secretive and if this is its winter ground then there could be a hundred of them in there and you'd never see one as in winter they need to feed less and they tend (where they feel pressure) only to feed at night. They should be moving off their winter ground soon if they behave anything like mine, or like those in the studies. If this is their summer ground, however, then activity should be increasing within the next few weeks and by the middle of end of April it should become relatively easy, with care, to see them feeding out around sunrise and sunset with the potential for the occasional one to move during the day.
It seems to me that a lot of people apply what they know about red deer to sika and while it is true in the most general sense it often doesn't work for specific behaviour. The following book, although very expensive, is well worth reading if you are interested. I would imagine a local library might get it for you, I managed to get a 2nd hand copy for a small fraction of the new cost:
http://www.springer.com/gb/book/9784431094289