The Rad Cap should have also been replaced with the rad, with a correct pressure for your system. They CAN leak, and malfunction.
Coolant recovery tanks can also spring leaks, which can lower the coolant levels, but usually not make them 'go dry',
Water Pumps - can leak around the shaft seal - not very common, but it does happen. When water pumps fail to pump (they go abruptly, like fuel pumps, not gradual), overheating is quite rapid, which yours is not according to your posting. Still - a shaft leak is possible, but not probable.

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