Don't think its a radiator issue since same along with the cap has been inspected by multiple mechanics and raised no concerns. Worst possible scenario would be a gasket issue, or it might simply be leaks elsewhere in the cooling system.
I'll need to inspect it a bit more thoroughly to see if there are any leaks, otherwise I just might have to take a trip back to the mechanic who overhauled the engine, head gasket might have to be readjusted to fix the issue in which case it would be best for the car to finish its running first.
Might need to try a number of things, will update on how the issue got resolved when it does.