Woof: From the stated facts, one senses that the problem is larger than just the caterer's seal and procedures for searching the truck. I would suggest that to achieve an appropriate level of security for aircraft operations, the airport security folks might need to work to a level of performance standards similar to that applied elsewhere in aviation.
One could could make a very specific list of points reflecting this perspective(others please add), such as:
1. The incident shows a failure to enforce existing security procedures - at least in this instance
2. Lack of any variability in the established checking procedures, makes them easy enough to subvert with long-baked plans.
3. Lack of redundancy in security procedures - so the failure of one component or the process means total failure & loss of control of the security mission
4. No closure of loops in the security process - lack of id checking upon exiting airside, for example - means missing an opportunity to catch mischief ex post facto, at least.
5. Only a single sleepy layer of security between the street and the jewels.
6. Evidently nobody is unpredictably but comprehensively policing the ramp and looking in on the players active there
7. The system does not seem to be at all oriented toward achieving a necessary degree of accountability in the security process at every step and at every moment of the day.