Ask yourself a lot of questions:
I begin by asking questions. The 5 Ws work for starters. An example set that might spring to mind from the seed you mentioned are:
- Who will tell the characters?
- What agenda will be behind the telling?
(help us, help them, keep this silent, be manipulated by me, etc)
- When will the request come in? Will it interrupt anything else?
- Where are the characters when they receive the information? Where will they need to go to get ready for whatever it is they decide to do?
- Why are the characters being involved beyond their occupation? Why should they care/not care deeply about it?
- How can things connect to past and/or future events?
I don't stop with a list like this, but this gets momentum going in thinking about the physical, social, and emotional environment between the seed and the players.
Once I have got this churning in my mind, I can start to think about what choices the characters might make if they begin an investigation, and the people that investigation would lead them to. That can lead me to an understanding of what locations and situations I may need to prepare as I consider where those people are, what they typically do, and how they might react to the characters when they meet.
Ideas are usually enough, but depending on how in sync you are with your players you may find more or less preparation to be useful in terms of locations and contacts.
All flesh must be eaten
Once I have all this fleshed out, I start thinking more deeply about how events happened, who is most likely to have been involved in making them happen - and based on who they are - how they could have done it. That leads me to a better sense of the clues that may have been left behind. Were they in a hurry? Do they know all they need to know to cover their tracks? Is everyone involved in the plan committed to making it work?
Let sit uncovered, then serve
I let this stew for a while (if I can) then review things from the top for the desired amount of coherence/confusion, the desired amount of freedom to act, and the desired amount of tension/time pressure.
The last thing then is to add players~