We don't know where the Pitstop is but I had been leaning towards the Coffeehouse in the Boboli Gardens--but that could be the site of the first cluebox instead.
I'm thinking the first cluebox perhaps. The Boboli Gardens, in my opinion, is too touristy of an area to have the pitstop. too many people around to have Phil and cameras waiting for the teams. I would figure that the pit stop, or faux pit stop if it is a TBC leg, to be at a more obscure out of the way place, e.g., a castle, winery, resort, etc.