It's been a while since I invested a lot of time in IC but check the following.

Is there a quest door to enter somewhere?

Is this one of the building which has no ground entrance but there is a bridge above from another building to enter?

Taylor_MB wrote: »

Enter directly via the ladders from your sewer base, not from the random access points in the sewers proper.

The ladders in sewer base will teleport you straight to the scouting mission location (2nd floor of building).

With "your sewer base" do you mean that base where you arrive when entering the imperial city from Cyrodiil?

I have been scouting in those irrigation tunnels that start from my sewer base, but I can't those ladders. The only ladders I could find where the ones that take me to the Temple District or the Arboreum District. Has anyone a map where they can mark the location of that ladder I am looking for?

When you first enter IC at your alliance's base, there are rooms to each side.

One of those will have 6 hanging ladders and a banker. Each of the 6 ladders will take you topside, one per zone, to a safe area.

There will be a questgiver there (the rogue if you're doing mainline, daily questgiver if you've completed it.) and an option to plant the flag (look for something resembling a banner.) Once you have planted the banner, you can go back down the same ladder to your main base.

This area is not accessible once you have jumped down from the ledge into the respective 'cheese wedge', short of dying and respawning if your alliance controls the zone, or returning (the long way, below) to your alliance sewer base.

Side note: There are tunnels going the other way, too, in side said base, that will lead you to vendors and crafting stations.