It's been a while since I've done this but here's what I remember.
You can use the last screenshot on the Egypt guide page; the tomb is under the house at the bottom left of that image. It's the house that's kind of by itself outside of town, the one above it is a vacation home.
The entrance to the tomb is in a shed on the property, you will need to get invited inside to have access to it (or sneak in if the house is unoccupied).
You need to deposit a bribe in a glowing chest in a room full of computers, I think the Sigil is in one of two other chests in the room.
After depositing the bribe you will get a new adventure to hack a HQ computer. That computer is in a room right next to the one you're in.