This page of our guide to Doom Eternal explains what is symbolized and indicated by a large question marker icon on a 3D map of the area. You can encounter question marks during the first mission of the single player campaign.
The question mark appears on the map after you get close enough to it - the example is shown in the picture.
Reaching a lot of question markers can be troublesome. Carefully study the 3D map and look around, for example, for breakable walls, climbing sites or wall-sticks that you can grab while flying in the air.
Each question marker is a unique collectible - it can be, for example, some kind of toy. Reaching each of the question markers is mandatory if you are planning to discover 100% of the secrets available in a certain level.
It is worth noting that all other collectibles available in a particular mission are also marked with smaller question marks. It's an excellent way of discovering how many secrets there are to find. In the further part of the game you can buy an upgrade of the armor, which allows you to "discover" all question marks. Thanks to this, you can find out what secrets to look for.