So here's the scenario in my head:

The Resistance is planning an attack on the Combine. So what else to do than put an immense bomb somewhere in the city? But the problem is... nobody knows where it is.

There are many ideas to choose from on how to RP it:
1-It would be kind of like a huge game of extreme hide and seek, where the combine put the citizens to work to find the bomb (the bomb would look obvious)and let the person who finds it first get a reward,
2-All the citizens could be locked down and only some elite combine allowed to look for it, while the rest stay in the Nexus (good RP opportunity for the Citizens),
3-Let the bomb go unnoticed by both the Combine and the Citizens... that is, until they hear the explosion 

This idea may be a little rough around the edges, and that's why I'll be open to suggestions/editing.

Thanks to CG for having this great server!