Is it possible that you have a small hole or a worn connection on the hose (small dia.) that goes from the expansion tank to the pump?

I only bring it up as I had mine (4.8iS) pop off while driving and didn't lose very much coolant eventhough I had driven 15km in 44c heat. I'm unfamiliar with the 3.0 so I'm not sure how your line are set-up.

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"When the Team Chief said.... You're trapped in a hole with nothing but a goat and a slinky, what do you do? Stubby said, I'm not sure but it won't end well for the goat...." ~(Overheard) Last day, Phase 3, Q Course