It still surprises me that certain things (a knife, boxcutters) go undetected when going through security.
For some reason, my carry-on seems to be pulled aside two out of three times. Honestly, I don't believe I carry anything out of the ordinary. I have to stand there while security personnel go through every item in my bag, and make comments. Last trip, an agent fell in love with my Neosporin/bandaid kit on a keychain.
I always wonder what they think they see that causes them to put my bag through a second or third time. Yet, another bag goes through (unintentionally, I'm sure) with a knife.
On one of my trips leaving from Tucson International Airport, I forgot that I had a half-empty bottle of Pepsi tucked in an outside pocket of my carry-on. When going through security, I was asked to throw it away. I sheepishily tossed it, and was a bit embarrassed because usually I'm really careful about checking everything.
ETA: To answer the OP's question, I probably would not take the item back to security.
About a week after I'd flown out of Tucson, there were news reports about a woman that actually got through security with a gun in her carry-on.
They found my Pepsi, but not her gun.
ETA: To answer the OP's question- I would probably not return the item to security.