The Register (www.theregister.co.uk) had an article like this some time ago. It turned out that the "headstones" were flawed headstones or rejects from some monument company that got used as pavers for a path or patio.

I'm sure there's no shortage of people who'd get a kick out of using headstones as pavers. And then the next owner just covered them up (or let them sink into the soil) rather than dig them up and dispose of them.

Next mystery!