How do you know that there's not a Necron Tomb City under Terra? Have you been there, or are you one of GW "Loremasters"?
Cheap insults aside....
Look at the facts that have been published about Necron Tomb Worlds and the History of Terra.
Near enough every tomb world is a desolate barren of a place, and those which do support human life generaly have countless stories of "metal monsters" that have been told for generations... as well as places on the planet that are considered haunted etc...
The probablity of a Necron Tombworld being on Terra is very minimal, given that mankind in the 40k timeline has covered the globe for over 50,000 years (from a civilisation point of view) and not once has any 40k fluff indicated that any tomb exists on Terra, not a single story or a rumour or anything.
Then take into account that the Necron tomb's generaly wake up with the slightest bit activity around them (various 40k sources have instances of mining operations etc... wakening them up so to speak)...
Aside from the massive amounts of activity on Terra for millenia, look at the Horus heresy... the crust of Terra was shattered by the attack and the oceans evaporated... if a simple mining operation can wake them up then how did the war to end all wars not wake them up?
Not to mention the Emperor of manking spent the best part of 38,000 years actively wandering the Earth guiding humanity... do you think its likely that a mechanicus exploration team can find a Tomb on a recently colonised Imperial world... yet the Emperor couldnt... with all of his powers?
Now for the rest of your post.
Also, Necron Tombships can pop out in any corner of the galaxy, an entire Necron fleet could be in Terra in a blink, greatly damaging, if not destroying, Terra itself. And let's not forget that the Void Dragon may be under Mars, which is relatively near Terra.
The Necrons do indeed have fairly amazing tech.... but the one thing they cannot do is just appear and instantly own the defences of Terra.
You are thinking of the 3-4 necron ships that ran
through the defences of Mars and tried to land.
The important part of that story is that they didnt stop... making it near enough impossible for the defences to target them.
As soon as they tried to land though the defences got a lock on them and did manage to destroy them.
Any Necron fleet that jumped into Terras Orbit would have to stop at some point, be it to engage the orbital defences or to try and land....
As soon as they stopped/slowed down they would be vulnerable... and Terra is the most heavily defended planet in the entire galaxy... The Necrons cant just insta pwn the defences.
And Terra has already had the combined punishment of all 4 chaos gods, plus near enough half of the traitor marines and their crusade fleets (including numberless amounts of demons and traitor "guard") pounding it for months... it survived then and is even more heavily defended now.
Why the VD may be on mars... dont click if you intend to read Mechanicum
Lastly the only reason the Void Dragon is on Mars is because the Emperor kicked him there.... its no indication of previous Necron activity.
And IF there is (which is highly improbable, yet it is possible) a Necron City in Terra, the Necrons can just pop anywhere on there, via their phase technology. Let's remember that Necron scout ships have easily bypassed Terra's defences, just think if a fully fledged Necron Tombfleet teleported right in front of Terra...
So what if the Necrons can just pop anywhere?
Their targets would be obvious... the Palace, the emperor, the astronomican... all of which are permenantly defended.
No matter how you attack those targets... theres always going to be hordes of IoM defenders.
Not to mention the Palace itself is heavily shielded
from near enough all forms of attack (enough that the shields lasted for the entire Heresy siege), the Necrons cant just beam into the throne room and kill the Emperor before you try and come out with that.
And no, Scouts ships have never bypassed any of Terra, that was mars and as stated they got destroyed when they stopped running.
Yes imagine if a Tomb world did pop up in orbit... it would be a hell of a fight. But you are throwing reason, established fluff and probablity out of the window in order to fit your views of the necrons. If the Necrons could do half as much as you think they can then they would be utterly invincible and nothing could ever counter them. And 40k would officialy end.