This exact same thing happened on my 2000 Outback. I found out when my AC stopped working accompanied by a flapping noise while almost home following a 8hr (each way) road trip. The flapping was my AC compressor belt, which had come off the compressor pulley.
I was lucky, the crank pulley didn't make it to the timing belt before I caught it.
I did a writeup on SubaruOutback.org: