Here's a follow-up in case it's helpful to someone in the future...

I followed Al's advice and opened the filter up. The sand was GONE! One of the 16 laterals was broken completely off. Two others had noticeable cracks. The broken one was probably just cracked last summer, because we'd had a constant but manageable trickle of sand into the pool. But when I fired up the pump this year, all remaining sand was dumped into the pool. And this filter holds 200 lbs of sand.

Customer service at Sta-Rite was awesome. The rep explained that their current laterals for this type of filter attach with a twist, but those from several years ago were glued in. For replacing glued laterals, they sell an adapter that connects the lateral to the manifold assembly. I didn't need any new laterals, just an adapter to re-attach the broken one, and some cement to seal the cracks.

Thanks for the help, Poconos and thepoolman1.