Keeping bombs out of the cargo bay is one of the things we seem to do very well, and has little to do with all the screenings we instituted since 9/11.
How many planes have been bombed by someone sneaking something into the wheel well?
I can't imagine it would be too hard to place a bomb in the wheel well of a plane parked in Nairobi, for example. The biggest issue would be avoiding the pilot's pre-flight check.
That wouldn't be to hard to do if you are a member of a ground crew that has access to the aircraft after the pilot is on board.
How many planes have been bombed full stop? As an attacker, why would you ever try to bomb a plane? Unlike a hostage-taking/hijacking where there are obvious advantages to being in a plane, if you just want to plant a bomb and kill some people you can do that much more easily in a building, or on a train, or...
How many planes have been bombed by someone sneaking something into the wheel well?