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Theo's body location is a note in Fallout 3.
It is given to the Lone Wanderer by Reilly in the Underworld's The Chop Shop after accepting to help her in rescuing Reilly's Rangers, who are trapped by super mutants on the roof of Statesman Hotel.
Reilly told me that Theo's body can be found on the second floor of the Statesman Hotel in one of the stairwells. There should be an ammo box nearby.