Are you riding or parking your bike on gravel or off the road somewhere, like an unpaved parking lot?
Its pretty hard for a nail like that to stand up and puncture a tire on pavement, unless someone put it under your tire deliberately.
Estranged wife, ex GF, recently fired employee... creepy neighbor?
Where I live I can take my trash to the landfill for $5. Have to be really alert in the parking lot, because people do home construction jobs, and drop nails and screws all over the place by the giant dumpers bins at the land fill.
Dont want to make you paranoid, but sometimes people are out to get you.