Try searching for the wayward ball bearings with a strong magnet. A nail sweeper is ideal for the job, available at many tool stores, often for cheap. Harbour Freight currently has one for $12.99. If you don't have one try borrowing from a friend, neighbour or sympathetic roofer. Back of a car speaker will often do. If you check the 700 FAQ here there's a brief note on reassembling the track after the ball bearings and rods fall out. Only time it happened to me I caught it at the first two ball bearings and one rod -had to check the other track to be sure how it went together.

Thinking of nail sweepers, I swore off working on my car on a gravel driveway after spending too many minutes with a sweeper locating errant fasteners. Found a lot of buried roofing nails in the process, even a few sharp nasty deck and drywall screws -for the sake of my tires and barefooted children I'm glad they were mildy well buried. I try to throw down a tarp now if I'm forced to work on gravel and worry the belly pan won't catch things from my fumble fingers or falling out of a socket wrench.
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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now