Do you let people know?  

Never thought of it but it sounds like a good idea. I'd hope someone would tell me. I always thought when you took your car in for an oil change they checked that stuff. Maybe it's only at dealerships. People are lucky there's someone nice out there like you looking out for them. :-)

I have done this a few times. More often than not, I'll see someone who has left their lights on or their interior light on and I will knock on a few doors to see if anyone knows who the owner is. I know I have done this a few times and my battery was almost dead in the morning .. I know people saw the lights on but they couldn't take 15 seconds to knock on the door or ask if anyone knew who owned the vehicle.

Any time I have told someone that their tail light or turn signal was out they have always been extremely happy. Around here they don't give out warnings for that, its a ticket no matter how much you argue, beg or plead.