It's unlikely to be the ABS. Subaru brake systems are robust and have few issues - this makes diagnosis tricky and guesswork of limited efficacy.
I would *very carefully* monitor fluid level and look under the car with a light and careful attention - following every line from the master cylinder all the way to each caliper. Look for a leak. It doesn't take much loss to cause issues. Even a tiny leak that's almost imperceptible in the master cylinder reservoir, and doesn't drip on the ground is tremendously problematic.
This happens on vehicles with rusty brake lines. The rust "scales" the metal lines and it doesn't cause a hole exactly - but many layers of scaled rust. It's very interesting looking - with no holes at all the fluid just seeps through like a maze through the various layers of rust. And it can be intermittent. Ambient conditions, rust progression, brake use - can cause it to normally work fine but sometimes find it's way through the right combination or layered rust maze.
The most common ones to cause this are the ones above the gas tank - they're exposed, never washed, can't be seen. The fluid very slowly just wets the metal lines/saturates the rust and doesn't drip. or it drips and just solidifies with 10 years of dust/debris on top of the gas tank and never pools enough to run down the gas tank and be visible.
But this is usually 1990's Subaru's with rust....not 2007's....but I can't see the vehicle so I dont' know what we're dealing with. This is the most common causative issue for what you're describing but I'm doubtful this is the case here due to how new this car is. If it was a 1995 Subaru in Maine I would almost bet money that's what it is.
1. Can you confirm there are no lights on the dash and haven't been for quite some time?
2. Has this car every been wrecked?
3. Has this car ever seen a major repair?
4. Is there any rust under the vehicle?
I agree with @brucep
, but if the poster can't distinguish between those cases, he should leave the job to a pro.
This isn't always that easy on asymptomatic vehicles. how can they diagnose something that doesn't exist? They usually replace by guesswork and get it wrong the first...or third...time.