Combing our driveway gravel yet again, with a borrowed 7-pound magnet, brought up the same sorts of sharp metal detritus as two other such forays had done.

Update: Each time I've done it, I've sent an e-mail and photo to the company that sold the gravel. This last time, I sent the WRONG photo..it was a plastic piece from the front of a car that we had found a few weeks back...and the owner of the company came up almost immediately to discuss the issue with us and complimented me on my work, clearly visible as he drove in. He said he'd send a crew (and an even stronger magnet) up to rake through it deeper than I could do with the magnet we had borrowed. We'll see!

Update, 11 March 2021: That hasn't happened yet.