There were times that it was close to the front, and not 100% outside the realm of possibility.

For instance tons of it was stockpiled in Africa prior to the invasion of Italy/Sicily.

Then come September there was a belief that Germany might carry out their threats of potentially using chemical weapons in Italy as Allied forces were driving up the peninsula.

In preparation for this a Liberty Ship the SS John Harvey was loaded with 2000 Mustard Gas bombs and sailed in a convoy to the port of Bari an Allied supply hub and major port on the East coast of the "Heel" of Italy.

But before it could be unloaded a Luftwaffe air raid struck the port, doing major damage to the facilities and ships on the night of December 2, 1943 and sunk 28 vessels and killed about 2k Allied servicemen, Merchant Mariners, and Italian civilians.

Amongst the ships sunk was the John Harvey, with explosion causing a release of the Mustard Gas. Nearly 600 personnel had been exposed to it, and due to secrecy the local medical units were not aware of the exposure and were only treating their burns and obvious injuries, and in the end about 80 of the cases of confirmed exposure died.

The Allied High Command also attempted to cover up the presence of Mustard Gas, and only months later would admit that it was even in the theater. Winston Churchill went so far as to even order all the causes of death changed from gas exposure to burns resulting from enemy action.

The Day of Battle: The War in Sicily and Italy, 1943–1944 by Rick Atkinson

This is a wonderful history of the often overshadowed and at times truly horrific Italian campaign.