Ghost in the machine...

I have a 2009 Prius Gen 2 with just a little over 100,000 miles on it. It is the second Prius I have owned and in both Prius's, I have never experienced the problem I had today. It goes like this:

At about 6:45pm, I left the house to take my Daughter to Karate practice. I decided to work from home today, so the car had been sitting all last night and all today up until that time. I approached the car like I normally do with my FOB in my pocket (SKS) and attempt to open the door several times to no avail. I try the unlock button on the FOB with no result, either. I finally use the physical key to unlock the door and enter the car.

I try to start the car and get no response. I pop the hood and make my way around to the front of the car (of course it's sprinkling) and while trying to open the hood, I notice the front horn is chirping faintly (almost unnoticeably) and the lights are dimly flashing. I'm not sure what to do at this point, so I disconnect the harness in the fuse box thinking to reset everything in hopes that whatever loop it was stuck in would reset.

Finally, I re-enter the car and insert my FOB physically into the slot, press down the brake, and hold the start button in for a few seconds, and voila - it starts up just like nothing had ever happened. I close everything back up and head off to Karate, come back out afterwards, and everything is normal.

From my reading here, it looks like my waning 3+ year old battery may be the problem. I plan to do the battery tests tomorrow listed in the weird-stuff-happening link I have seen.

But I did not run across any similar problems, so I wanted to get this out there in case anyone else has experienced this and it might be something more serious. Thanks in advance for any advice!