In Head of State quest, Hannibal asks me to meet him outside the memorial, but when I go there he is not there. Where is he?

he is usually by one of the metrostations in DC.

I found him right where he was

One would expect him to be there by the time you fast travel to the place yourself but he and his group seem to go very slow, do not attempt to escort him or chances are he'll get killed or something, just fast travel there and wait a couple of days there until they arrive, you'll get your reward, go back there some time latter and they will have restored the monument and build a camp in there, here you can perform some business like selling the Lincoln related items from the museum to the girl building him a shrine.