This page provide information on the final actions that you have to perform to complete mission "A house Built on Sand". You will still have to obtain documents from Matthieu Mendola and escape the map.
Obtain the documents
Apart from murdering your targets, to complete "A house Built on Sand", you also have to obtain secret documents, which had to be handed over during the meeting. The documents are on Matthieu Mendola. You cannot, e.g. pickpocket them from the target and instead, you have to obtain them after Mendola is dead. The documents appear in the spot where Mendola died (as shown in the above screenshot) and if you decided to kill the man at a distance (e.g. to snipe him down), you will still have to get to the corpse to get the papers. The good news is that nobody will take interest in the documents.
Note - If you cannot find the documents to, access your map. They should be marked there (after Mendola dies).
Escape the mission area
You can leave the mission area only after you assassinate both targets and after you have obtained the documents. There are four main exits (M10,12). Apart from that, you can also escape the area in a small van near the starting point in the Southern part of the map (M10,13). If you want to use this route, you need car keys that you can find on one of the coffee tables in the Southern part of Shisha Café (M10,2).