A failure in the anti-lock brake system (ABS) causes the ABS pump to continuously run, which will eventually wear down the battery. An improved ABS relay is available to solve the problem.
Car Problem Reports
Subaru Legacy Failure in ABS causes pump to run continuously and wear down battery
Subaru Legacy Problem
Average Mileage: 189,000 mi (135,000 mi - 227,000 mi)
Visitor, 1994 Subaru Legacy, 205,000 mi
I just figured this out by reading THIS, but I inadvertently fixed the issue when I unplugged the ABS unit because it was pumping air into the brake lines and leaking.