It must be unnoticeable.
I was flying a Valiant tanker in the early days when Wattisham's Lightnings were doing their first refuelling training. No1 did his practice engagements and then took a couple of thousand of fuel. His No2 came astern and there was a long period of inactivity. There than came a bleat from the No2.
"You'll never believe this; I've brought up an aircraft without a probe." Followed by a mad dash to get to Coltishall before it went quiet.
His original aircraft had gone u/s so he had rushed into the line with a 'give us another one, Chief' and got one which wasn't fitted with a probe.