The vent was the "Donkeys Dick" that is directly above the chutes Noddy Cap, they used to vent from them when refuelling if the valve was a bit dodgy, sometimes really badly and in flight they could be a give away as to where the Jag was as you could get a white streak of venting fuel behind them.
see
https://www.airteamimages.com/sepeca...592_large.html
You could also jettison the drop tanks that were a standard inboard pylon fit.