Given that you have no stored trans codes it’s not likely to be the output shaft sensor on the transmission. Could be a faulty wheel speed sensor, they do fail and will set the ABS light. Every now and again when the conditions are right, rainy or very high humidity, my 07 will trip the ABS idiot light. I assume in my case that the potting on the back of a wheel speed sensor is letting moisture in. It always goes away after one ignition cycle or once the weather drys out a bit. I have never nailed it down due to the fact that without a Subaru scan tool or something like a Snap On Soulus, which I do not have ready access to, there is no way to access the ABS codes/data. I’m not sure if Subaru monitors ABS pump pressure, but your issue may be as simple as low brake fluid. Just a theory, but easy enough to check the fluid level. Or perhaps take the car to a dealer or a competent mechanic with a good scan tool to check the ABS system.