- leave hand written note on desk/in pigeon hole
- send IM/text message.
- put topic on a "list of things to discuss i next meeting"
- phone call/face to face
You should choose the first that fits the urgency of the situation. The idea being to maximize trackability and minimize disruptions. In the current Slack culture, I actually put IM just before phone call, given the expectation of quick answer most people have.