One thing I was taught early on, when I started working with sensitive material: when you get up, even to get a cup of water, you lock your terminal. The enemy is always around.
Usually, it's no biggie: with Windows, you just click the Windows Key + L and the OS is locked (similarly, if you're an aspiring pianist, you may go for ctrl + alt + del and then click "Lock Computer"). But sometimes you forget. And if you forgot in a room full of tech people, as once happened to me, you pay the price.
One colleague (I now know who) opened my mail program, and wrote an email (full of BS) to our CEO. He left the mouse hovering over the "Send" button. Luckily he didn't click it - and neither did I
I've learned my lesson, but many people around the world haven't yet. And their "friendly" colleagues are there to remind them. The art of mucking up a person's workstation in his absence is called "Goating" - because originally people would put a picture of a goat on your screen while you were away (don't ask me why).
It has advanced a bit over the years. Other than changing your background, fonts, language, send emails to distribution lists etc., some people resort to smarter tricks. Such as replacing your desktop with a picture of your desktop (minutes of fun to watch), or invoking Clippy, the annoying Microsoft "helper".
Read and get some more ideas here. And don't get near my laptop!