It is (supposedly) most likely at XXX, since the client will use old coins/transactions when sending 5 BTC from XXX to YYY.
So, this means it is client dependent?
I'm not convinced that this the proper way to trace "my coin"
Yes, it's client dependent on what coins it chooses to spend. If you have a 50BTC input and a 1BTC input, then tried to spend 5BTC, you'd 100% spend the 50BTC input, as, 1BTC doesn't cover it, it'd then return 45BTc change, and, you'd have a 45BTC input and a 1BTC input, unless, specifically told otherwise, in which case it'd use both (50BTC input & 1BTC input), and, get change of 46BTC, as, a singular 46BTC input, although, no client would do this automatically as it'd reset the priority of the address, lose anonymity, and, be absolutely worthless.