If you're in the Philadelphia area and you've lost a lag bolt, I found it.
Right between the tread block on my Dr Side Rear Tire of my 09 MCS Conv.

I got the TPM warning this AM. I figured tires were down a little w/ the cold snap.

Got tot this tire & found the culprit. Rear won't hold pressure above 30lbs, so I figure the bolt penetrated.

These can't be patched, right?

W/ 6,000 miles on the car, is it safe to replace just 1 tire?

 

It can be patched, need to find the right shop, but they will patch it.

If you need to replace, make sure your fronts are matched, but 1 tire can be bought unless the wear is excessive.

 

IMO - only an honest tire shop (I hope you have one near you) can say whether the tire can be repaired.

With only 6,000 miles, unless you drive like a maniac, I would think a new tire in the mix of the other three would not present a problem. Again, an honest tire shop will give you the right answer.