Those guys at Meineke sound like amateurs. I'd pull the wheels and inspect the calipers for leakage. Check the brake hoses and lines. Check for leaks around the ABS. Look around the master cylinder and fluid reservoir too. It could be leaking inside your brake booster, so you won't even see the leak, until it's too late and your brake booster as well as your MC need replacement. Remove the cylinder from the booster and run your finger along where the piston enters the cylinder. It should be dry. If air is getting in, that means fluid is going out. It ain't magic, it's just a matter of finding it.