i know that others who have completed this series of posts have talked about their mobile (cell) phone as being the thing they would never leave the house without. But as recently as Saturday, when my phone was charging, i went for a hair appointment without my phone. Also if out with others i am happy to leave my phone behind when say out for a walk. So maybe that is not the correct answer for me.
i think the answer is probably my keys; front door and car attached to each other on a key ring. Even if someone is home, and i know they are not leaving the house, i have a morbid fear of not being able to get back inside. This is because i have previously left the house, shut the door and then not been able to either get into the car or back inside. On that fateful day, my neighbour (who has a key) was in the middle of house renovations and couldn't find our key. Hubby had to return from work so that i could leave. Of course on that day the phone came in useful. Since that day, i have made sure i always have my key, even if it means carrying it in my hand (say if no bag or pocket).
This sounds very dull, but that is how it is. Lets hope the next one of these is more interesting!