doorknob

Traces of the chemical weapon were later discovered on the doorknob of Mr Skripal's home, where police believe it had been planted in a "targeted" attack.

It was subsequently discovered that they were suffering from the effects of a chemical which had been left on the doorknob of Mr Skripal's home.

There exists something in medicine called the “doorknob phenomenon.”

Think of it like cancer leaving a fingerprint on the doorknob of a crime scene.

I step into my slippers and twist my doorknob; it’s not locked anymore.