Features of the HHR you may not have known
 

Here are a few features that I discovered the HHR has.

First if all, the center mount brake light dims the longer your foot is on the brake. Do a quick tap after it is fully dim and it will still remain dim, drive a while and do a quick tap and it is bright again. Possibly a feature to not bother people behind you in heavy traffic.

The windchill wipers change speed based on the vehicle speed. You can notice this when you set the wipers on a certain speed and then take the car above 50MPH the wipers then change to a faster speed until you go back below 50MPH. Great for changing from freeway to street.

Lastly, if you are driving from normal temperature weather to a place that drops below freezing like 28 degrees F, the dash will chime and ICE POSSIBLE will be displayed.

Anyone noticed any other features?

2 and 3, I agree with. I have other cars with those same functions so I was ready for those.

No. 1 about the 3rd brake light. Not sure about that one. In fact, when I was installing my SS cutout sticker in the 3rd light lens, I didn't notice any dimming occurring. And yes I had the brake lights on and off for extended periods while I was "centering" the cutout.
Maybe yours has non-stock bulbs ?

I found that there are certain places I go regularly where I put the Auto Trans in I for compression. This one spot I go regularly runs up to and holds right at 30mph with foot off gas pedal and that is the speed limit I need, but if I just barely touch the throttle .. it releases to a higher speed. It allows me to ride the 1 mile stretch without touching the brakes as I watch most of the cars in front with there brakes on. Kinda neat feature and I use it.
Just found that by accident.

Silverfox

Also never seen the third brake light dim on either HHR of mine. And I wouldn't want it to. If I'm at a long light no one behind me and it dimmed someone might come up and run through me. :O

I also added the SS sticker and had the brake light on for a few minutes at least to get it dialed in and it was always bright.

I really doubt this is a feature. If it is, we should find that as part of the wiring diag.

Are the stock 3rd brake light bulbs Halogen? I always thought they were just standard bulbs and any replacements I've put in have just been standard bulbs.

My OEM bulbs were just 921 incandescent

I think the standard is incandescent. Halogen is available.

Wiring diagram would not show anything since the BCM controls it.

Sorry, yes standard incandescent.
I now have LEDs and put the cover back on.
You will have to ask someone that you know that is following behind you one day. I will do the same with the LEDs.