My 2-pennyworth on this is: Don't wear a squawk of a unit you are NOT working. Someone may be trying to coordinate your track on the basis of that squawk.
I was subject to an MOR (MANY moons ago) when Clacton sector 'handed' an aircraft to Eastern Radar (0232 - yes, I remember it THAT well!) as it left CAS at FL160 for Mildenhall - directly on track for the Upper Heyford (in the open FIR, btw) stack ( I said it was many moons ago!), base hold level FL150.
There was one F-111 in the hold at FL150.
I expected Console 3A at Eastern to call to coordinate the track to 1000ft vertical (Eastern 3A had just handed me 2 others inbound to the hold, but I had stopped them at FL 170 and 180 respectively to permit the transit at FL160). In the absence of any coordination from Eastern 3A, my nerve broke first and initiated coord:
"My 6131, in the Heystack at FL 150, maintaining; my 6132 and 6133, joining the Heystack, FLs 170, 180 to maintain, against your 0232 to maintain FL160 until clear of the Heystack, agreed?" The response?
"Sorry 13, not working 0232 yet, still with Clacton sector - can't coordinate."
I'd wasted what little time was available trying to coordinate with the wrong agency.
The C-130 (for t'was it was) then ploughed through my hold without a bye or leave with my 3 tracks star-bursting around the sky in a (vain) effort to 'get 5' (miles).
The 'kicker' was that the Clacton Sector Controller then MOR'ed me
for not coordinating with him, even though his track was 'off-route' in the open FIR, where the onus was on the Civ side to initiate coordination with the Mil side, wearing the wrong squawk, whilst transitting through a known active, holding pattern! The paperwork, transcripts, went on for weeks!! And led to more than a few sleepless nights.
I re-iterate guys: NEVER wear a squawk of a unit you are NOT working, especially outside of CAS.
PS - shouldn't it be 'leave CAS by dissent'?