Today I parked on a small hill that was probably about a 25% - 30% grade. I used the parking brake as always and did not have any issue when I first parked, even just leaving the car in park without parking break. When I returned to my car about 2 hours later, removed the parking brake, the car began to roll down the hill while making a "clicking" sound; almost like park wasn't working at all. What causes this? Is it something to be concerned about?
What causes this?
Automatic transmissions use a gear-locking lever called a pawl. When you put your car in park, the pawl pushes down onto a cog looking gear and locks into place on it, preventing any movement in the transmission.
Here is an excellent explanation of the mechanics involved.
Depending on the age of the vehicle and the kind of wear that it has seen, it is possible that the sharp edge that keeps the pawl inside the grooves of the ring has worn down. I don't think this is very likely, and it may have more to do with a one-off failure (if you haven't seen the issue since this).
There is still a chance that the pawl or the spring that pushes it onto the ring is worn though, which would help explain the clicking sound you heard. As more clearly shown in the picture of the answer linked above, there are sections of the ring that separate the notches, which would make a clicking sound if the pawl was not engaging with the notches. Kind of like a skipping stone, the pawl may fall into the notch just a bit, and then be kicked up again.
Is it something to be concerned about?
It depends on whether or not this is happening regularly. If this is the first and only time that you have seen it, I would take it to a hill of a greater grade and play around with it. Try holding the car on the hill using just the park gear. Do not get out of the car with it just in park if there is any doubt it may slip. You do not want the car to roll back on you or run away down the hill.
If you have any doubt that it may fail again or just for peace of mind, take it to a transmission shop.