Problems with Electrical Connections

Hey Everyone!

First-time poster and inexperienced hybrid owner here. Reading these forums has helped me out a lot with several Prius issues already. Thanks guys!

So... I attempt to turn on my car to drive home, and it is electrically dead.
I try to jump it with a power pack. I hear the car systems ticking over and the pack appears to be charging the battery, but attempting to start the car results only in clicking, along with the brake alternating between stiff and loose.

I check the terminals by the fuses in the front of the car, and it returns under 4 volts (should have done that first, I know). I check the actual 12V battery, and it is around 12.6V. I disconnect and reconnect the battery, cleaning the posts and connections (they had no apparent corrosion and only a little debris on them). The car starts up. I am able to drive it and start it up fine, at least so far.

I want to be complacent and say that this was the result of a bad connection at the posts, but I am concerned that there might be another intermittent problem going on as well. My Prius has previously had problems with the battery draining for no apparent reason, but I was unable to repeat the symptoms. What can I check to make sure that something else isn't going on (like the battery power being intermittently drained by something before the ignition)?