In 'Menace From The Deep,' Walker's name is mentioned in the rota to keep watch outside, yet it's Sgt Wilson who's able to apply the necessary skills to remove the chocolate bars from the machine, something Walker did in the TV series. In fact, I don't think you hear Joe's voice at all. Seems pointless mentioning him on watch duty.
I think its the "trick" to have the listener imagine that Walker is there. When you think about it -in many of the early episodes several regulars were absent as they were trying to keep costs down on the production. However, the storyline still requires the platoon to undertake scenarios, not just 4 or 5 members - all of them. So, its quite common for captain Mainwaring or Sergeant Wilson to give out an order, or make a comment to a character who may or may not be present in real life, but in your minds eye they are actually there. This happens to Frazer, Pike and Godfrey too over several episodes.
Although its interesting in the fourth episode, Jones refers to not having seen Godfrey for a while!