It's fairly common for radiators to leak, usually the plastic cracks. This wouldn't be surprising at all. I hate to guess without seeing it in person, but this is by far the most likely scenario.
Radiator shops, if available, can test them for leakage. You could try pulling the radiator, closing off the top radiator hose inlet/outlets, turning it upside down and filling it with water to see if you can find the leak. But it may not leak without heat/pressure.
Or try the UV light kits/technique, but I've never done that.
I've installed probably 10-15 $100 new radiators from various sources and never had issues. If another 15 years of ownership is likely a new Subaru OEM radiator probably makes sense.
I'd prefer Subaru hoses.