If he/she is friendly, you should scoop her up and bring her to a shelter. But maybe first get a discount rate on vaccines and nueter through Friends of Animals (Conn, I think) or any local other place that will give cut rate. A shelter will be more happy to take her/him if you take care of those things plus testing fiv/luek.
The problem with just leaving her/him there is breeding, sickness, etc.
If not friendly (feral) you're gonna have to decide if you're willing to pay for all this and decide whether or not to put the cat(s) back. Not a good idea unless you're willing to feed them the rest of their lives, and in cold weather, make strofoam shelters with hay, etc.
If she's friendly, yeah, she may be put down if there is so much as a sneeze. But, it is better that freezing to death and starving to death- both are very painful.
If feral- unless you can find a shelter to socialize her with foster care (adults are hard...) she/he will definatly be put down (I guess- unless you are willing to socialize). But again, it's more humane than dying a painful death and even more humane that living in a cage the rest of his/her life (in a "no kill shelter" that keeps a kazillion cats cramped and miserable because they are unadoptable- they get repeatedly sick and live in horrible quarters- unless you can find one that doesn't - there's certainly no humane no kill shelters around Boston!!!
Put down a bunch of hard food, it doesn't freeze as quickly as moist etc. Just pour out a huge bag of it every couple of days and don't forget water. They can't rely on puddles because the dust and crap in them is bad for their lungs and stomach.
Keep me posted re: post office cats!
(I worked on a colony behind the Peabody post office for a year or 2 till they just disappeared (died, I suppose). But me and my friend got several litters of kittens out before they went feral and the post office staff were grateful for our intervention!
It's always nice to see the little ones get adopted out!!