Mission 2 (Occupied Residence) in Sniper Elite 5 contains only 1 side quest, but it may cause problems for you, as it concerns searching for stolen antiques - collectibles. The walkthrough contains detailed instructions on where the stolen antiques are and how to get them.
Additional note - Antiques-secrets can be found in any order. This side mission will appear in your journal after you obtain the first of the three available collectibles.
Antique #1 - Soldier Statuette
The first artifact is carried by the sniper hiding inside the building to the north-east of the courtyard of the chateau. This place is shown in the pictures above.
It is best to sneak inside by walking next to the walls. Go to the upper floor, eliminate the sniper (e.g. takedown) and search his body to recover the stolen antique.
Antique # 2 - Group Statuette
The second artifact is inside the chest in the southern part of the chateau. You have to visit the top floor of the building and look into the room presented in the pictures above. You can use a lockpick or bolt cutters to open the chest.
Antique # 3 - Old Man Statuette
The last, third artifact is in the bedroom in the western part of the chateau. Explore the middle floor of the building to get to the right room. Start by locating the document lying in front of the bed - shown in picture 2.
Now interact with the painting on the wall to move it and reveal the safe.
Since you obtained the combination for the safe (by reading the document), you can open it without using explosives. Inside you will find the last stolen item.