It's the Martini shot, and all you need is that extra Flex Fill so the crew can wrap for the day and grab a few local craft beers, but unfortunately, it's no where to be found. It's obviously not in the small pelicans because it won't fit in there, but that's about the only lead you have. Now, everyone is staring at you, waiting to roll, and you're going through all twenty one cases in hopes that you didn't leave it in the production vehicle that's parked on the sixth floor of a parking garage twelve blocks away.
It's the AC's responsibility to manage gear, and the best way to do that is to have a place for everything.
Each company and each project is going to have different gear needs and casing options, but it's important to create a system that makes sense to you. I like to create grouping and tiering systems for all my gear. It sounds complicated, but once you create the schema, it's very simple and very quick. For example, Group One would contain all camera-related gear, such as camera bodies, lensing options, bricks, monitors, etc. Then, I create a tiered hierarchy of related gear, so Case One would be the camera body, while Case Two has the lensing, and Case Three has the Odyssey 7Q monitor. It allows me to remember where things are in certain places based on their proximity/importance to the camera. Then Group Two would be lighting-related cases; Group Three has grip-related equipment; Group Four is Sound, and so on...
Also, it's just as important to log all of your gear in a spreadsheet or list. The spreadsheet should be broken down by case and what each case has inside of it. This way, there is a record of the gear that was brought, so you if can't remember where something is, you can pull the list up and check (I keep my iPad with me, which contains the gear list and camera manuals on it)... I've got a quick draw on finding gear (and camera solutions).
Depending on where I'm traveling, I also like to tape a short list of the contents on the outside of each case. If you're traveling internationally or in very public places, you may not want to promote that you have a Sony F55 with the with hundreds of thousands of dollars in lenses inside, but if you're on a closed set, it can help help you find things quickly, as well as giving you the added bonus that there is a guide for where everything goes at the end of the day. You can also place that list inside the case if the location is sensitive (I like to to put it above the top foam).
Let's be honest, we can't expect for our brains to work perfectly after a 36 hour travel day or 14 hours in the Southwest Sun. Your questions have answers, so make sure they are written down and logged. A half hour on your prep day can change that feeling in your stomach that you forgot something on your 15 hour flight to India or help you find that cheeseplate (and help you get to the cheeseburgers) at the end of the day.