I'm sure someone mentioned on here that a 142 once went missing for a while and turned up a completely different depot to where it was supposed to be!
I'll see your 142 and raise you an entire rake of Mk 1 stock...
Stock for a railtour from Euston to an open day in the East Midlands; ended up running 4 hours late, much of it because BR literally forgot where they'd stabled the stock overnight. It eventually transpired it was just half a mile down the line in Downside shed.

Then one of the two locomotives on it was dud so it lost more time...

The result was a reduced time at the open day, a promised branch or two being dropped from the itinerary and a free compensatory railtour offered to the participants in order to get the branches in. Then they messed that up too because of engineering on at least one of said branches on the free tour date... :roll:
Anyway.... back to the 700s... it was probably pretty much the same thing; a question of the vehicle being shunted somewhere for the duration and then overlooked later?