Seems odd that it wasn’t even in the old ACP(19?) from before AP 818 was adopted.
Anyway, I’ve asked for it to cease until someone can provide a reference from the AP for it (and I’m feeling confident as a Ctrl + F for “cadet” produces zero results).
I had a similar question come up recently so thought I’d double check 818.
Neither the lesson on sizing nor the section on squad drill mention the last person calling out a rank or sir/ma’am.
Correct, it’s a made up thing
It does, however, clearly indicate the end of the numbering sequence, i.e. there isn’t another cadet who has been caught out. 
Add it to the AP!
Is it an army thing? I remember being taught it as a cadet 30 odd years ago.
Despite being air cadets we were all taught army drill & parachute regiment drill at that as the SSI was an ex RSM from the paras.
Something that has just carried over particularly for fire drills etc?
I think “last man” is an Army thing, possibly even “last man on parade”, but I never heard “cadets on parade” until recently.