There is a gusset between the left hand chassis side member and main cross member, just below the starter motor. It makes the perfect shelf for those nuts/bolts/washers etc small bits that you feel and hear disappearing but don't hear hitting the floor. What's more, it's usually covered in thick grime and holds very efficiently what falls from higher levels of the engine compartment. Every time I remove an engine I am surprised of what is stored on this gusset!
On the same topic, I did some work on a two-stroke Saab 96 once, and was surprised to find that, what looked like a full engine / gearbox undershield was actually a welded part of the body / chassis structure, with only a small hole for draining the gearbox oil! For the rest of the day I had to be triple cautious not to drop a part off my hands!