First, you are too polite if you actually drop everything to explain what's in a bag in your trunk. The person who asks is nosy and really it's none of their business. The guy who tries to open a container in the trunk of my car will find the lid closing on his arm, head or whatever appendage in the way of me closing the trunk. But back to your question about urban camouflage...

I keep stuff in drab to black duffel bags and sometimes in very toolbox looking containers. People see a toolbox in your trunk they think "tools". I won't open the box.

The stuff in my trunk is long term, a bag for the office rides on the front seat.