I think it very plausible that it could have been the serial attacker who did this, and Condit was not involved. Last day in DC, she looks up and visits an historical sight that maybe she had earlier heard about(a liberal interested in historic preservation is far from unheard of). Goes for a walk(not jog), early May isn't particularly warm, so that explains no change of clothes left at home.
However, finding 'no keys' raises my suspicions. If there was a prolonged struggle the keys could have been lost away from where she was buried, but it is unlikely the attacker would have dragged very far before burying/hiding her body. So if an extensive search of the nearby area(simple metal detectors that those beachcombers used to search for buried treasure) fail to reveal the keys, then likely they were purposefully taken. Why would a serial random attacker take them, since she was carrying no id? Yeah, he might have taken her goods and keys, and then once away from the body searched for any info that would reveal an address, but then proving useless would have discarded the keys. So again, a thorough search of the area should reveal them. Unless he tossed them in a garbage.
Hard to tell yet, but I wouldn't rule anyone or anything out yet(well, ok, maybe the Chandra is a kidnapped love slave in Arabia speculation can be tossed).