Hot water even though valve to tankless water heater shutoff?

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Noticed the shutoff going into my tankless water heater was off the other day, BUT I still had hot water throughout the house for weeks! I have a Navien tankless water heater and use the built in recirculation pump with a two branch dedicated return recirculation system. I can't see how the cold pressure was able to get into the hot side of the system unless it was able to leak from the cold side of my thermostatic valves to the other. Is this possible? System is brand new with ball valves only, so i don't see it being a leaky shutoff. I've also traced plumbing to make sure there are no cross connects. Anything else it could be?
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