I am a member of my county's SAR unit. Since we are on call 24/7 and often have to deploy directly to a field command post, we are required to keep all of our equipment (from a change of uniform to extra batteries) in bins in our vehicles. In my vehicle I also keep a waistpack, a daypack and a larger field pack. Each pack contains the same basic package: food packet, water, personal FAK, flashlight/replacement batteries, heatsheet, whistle, sunscreen, insect repellent, knife. When I arrive in the field I decide which pack to take and then add to the basic package any additional items from my equipment bin that I anticipate needing (headlamp, rain jacket, waterproof gloves, additional food, whatever). If we're looking for a lost child or a dementia patient in a suburban setting, nobody (at least after their first search) wants to hump around a full field pack loaded with enough gear to work unsupported in the wilderness for three days. I guess the key is to be clear on what you are preparing for. A one-size-fits-all bag might not be the way to go.