Sounds like a leaking head gasket. Wouldn't leak externally and would drain the reservoir... Any over pressuring/coolant water spots near the reservoir? I am more familiar with 6.0L powerstroke issues, but you never know...
2009 Expedition E.Bauer 5.4 Family Wagon
2002 F150 XLT SCab 4.2L 4x2 (Nephew's)
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