As it happens in a lot of our work, we become use to it or we get used to a certain way of doing things. If you keep an open mind, you will always double check your work or have someone else help you to do it. This will save you and the Agency embarrassment litigation later.
As I remember, there were several instances of "loaded" vehicles that were not completely unloaded of the contraband. Not all contraband is cocaine, marijuana or heroin. Sometimes there are steroids, pharmaceuticals, like medicines and "Viagra", even FDA items and "Illegals" of course, guns, currency, and even "mole", etc... The list is endless or as long as your imagination.
As the story goes, there was a van loaded with marijuana in the side panels. I know, I was assisting to help unload this mess while the seizing inspector was doing the most important part of the job, his report. After the inspector thought he was finished, he started to weigh the packages so that he could get his FDIN (federal identification number for his seizure) Every "significant" narcotics seizure gets a special number assigned to it for statistics which may be included in the national averages or other important statistical data. There was over a 150lbs. so far under the current weight.
However, as the experienced and meticulous person that I am, I wanted to make sure that the lower "rocker" panels were checked because I thought the sound of these lower panels were too "solid" sounding. Believe me, after a few thousand vehicles, you can determine what sounds normal or not.
As I was double checking the other inspector's work to make sure that there was no more contraband left, I noticed that there was definitely enough space to get additional packages. It was all very well done and "welded" into it's place. I went to notify the seizing inspector of this find and he thought I was kidding. I insisted that I wasn't kidding and to go look for himself. He didn't want to believe me so I had to do the next best thing to get his attention. I then pulled out another "blank" seizure/incident/arrest cf151 form so that I would get credit for the narcotics I was about to seize. That got their attention.
The inspectors didn't want to believe me and the Supervisor thought I wasn't serious. But what was I suppose to do? Let it go? You have to make sure all of the contraband is removed. Under the law, Customs and other law enforcement agencies are allowed to conduct another search for such occasions. That way, all of the contraband is accounted for and there is no question about where the remainder went.
There is also a current case in Southern California where a person bought a government seized vehicle and it was handled by more than one agency. It went from DEA to Sate and locals. The driver still had over three packages still attached to the underside of his vehicle near on the gas tank. For the life of me, I can't see how any narcotics interdiction officer could miss it, but it still happens.
As is stands now this particular case caused the victim to have his house searched and all of his assets seized because of the "foul up". The probable cause was based on the accidental oversight of the leftover contraband. Imagine that! The search could go on indefinitely. You could leave a little behind and go on to seize the next victim's car, house, property, etc.....
I think that the biggest problem is accounting for all of the contraband when the bad guy claims there was more. Then you have another "French Connection" situation. That was where the heroin was removed from the evidence lockers at the police station. It was pure Asian white heroin, worth millions.
So you have more than enough reason to make sure that all of the contraband is removed. Integrity and self satisfaction that you got all of it.