Coolant leak?

Most of the time it will be the water pump or thermostat housing which is at the bottom of the engine front, but the coolant can leak from the rear of the engine as it travels downward. To be sure we can do a pressure test and then raise the car up and while using a flashlight see where it is coming from which might be under a cover so that it may need to be removed.

https://www.2carpros.com/articles/radiator-pressure-test

And this guide can help as well:

https://www.2carpros.com/articles/jack-up-and-lift-your-car-safely

Here are the cooling system diagrams and how to change out the water pump in case that is the problem. If you have questions about something you see, please upload an image or a short video. Check out the images (below). Please let us know what happens.

Yep, that would be better, that way I can see what's going on.

Hmm, yep that is at the rear of the engine. Can you look above the bell housing or on top of the engine with the air cleaner intake boot covers to see if something is leaking down on top of it? With the engine off, use a small mirror on a stick and see if you can tell where it is coming from, I have seen coolant passages that will transfer the coolant to the rear of the engine if the water pump leaks. So, if you see an open port that coolant is coming from, I would say the water pump is leaking. If not, the transaxle will need to come out to tell where the leak is coming from. I will put this out to other experts to see if they have seen this problem before. Thanks for the videos, they help.

That engine isn't running too well, but look at the back of each cylinder head on the rear of the engine for coolant leaking out of the head gaskets. I don't think Subaru will ever get their head gasket issues under control. There are smaller hoses that direct coolant to the throttle body as well, a pressure test of the cooling system would be best, you can rent a pressure test kit from AutoZone for free, just need to put the money down for it and you get it all back once you return the tool. Just follow the instructions with it.

Very likely the head gasket. They like to pop at the outer passages.

Yup Steve is absolutely right, during the early-mid 2000s we had a huge recall on head gaskets at the dealership, we were doing a few a day for a year, they leak coolant and oil out the sides of the cylinder heads, there was an updated head gasket (thicker one), not sure it applies to a 2015.