'Casing' a jump. What does it mean?

Quick google search and as far as I can see it means coming up short on doubles and landing across the 2nd double instead of on the backside of it.
Is that right?

I think its where your back wheel clips the top of the landing rather than landing smoothly on the down-slope. I do it all the time 🙂

yeh.

its what happens when 90% of mountain bikers claim they have jumped something.

Yeah pretty much.
Refers to a very hard landing – typically landing on the peak of any jump in place of the downslope.

comes from landing on the 'crankcases' from the days of Scrambling (Moto-X)…

Yes, I think it comes from MX – where bikes have "cases" below all the mechanical gubbins.

Staking out a potential target building, noting any surveillance/security provisions, easy entry/exit points, where items of value/interest are stored, gathering information useful to facilitate any impending raid.

Eh?

A 'jump'? What on Earth are you talking about don't be so silly. 🙄