A few reasons why it may be on,
1. The abs sensor has packed its bags, Not a cheap sensor either so see if you can source a working one too put in before buying one just too check.
2. Abs ring could have worn and be no good, Once again you can not get the abs ring on its own and you will have too get the full cv joint for that side, Few pound for that.
3. you dont want this, the abs control unit could be gone bye bye, Big bucks but i'd say get a second hand one of any of the lads on here with car for scrappage and you will be sorted cheap and you could change it yourself as its not a really big job.
4. Wiring too the wheel could have broken somewhere so you will need too strip down and follow the wires all the way back till you find a breakage.
My advice is go too a diagnostics place and see what the machine tells you, Ciaran (Rs-Ra) Dave legacy have these machines and can tell you, also any subaru garage. And start from there and see what the diagnostics show up, You will not get any fault codes in the car if there is a ABS problem.
Best of luck and let us know your outcome.
John