I joined a group called Random Acts of Zentangle who encourage you to leave a tangled tile for someone to randomly discover, with a little explanation of what it is on the reverse.
And after finding a suitable spot to for your little piece of art, you then photograph it in situ.
I chose a Circle Line Tube seat late at night for my first RAZ. I boarded the train where it looked pretty quiet, placed my RAZ propped up against the window above some empty seats. After my guerilla photography, I leapt off at the next station. I hope someone spotted it before they cleaned the carriages.