Is the smell of feces, without its presence, a nullifier of ablution, and is it obligatory to search for this smell?

There is no need to inspect the anus or search for a smell from it, as this is incorrect and may lead to obsessive thoughts. The wind (gas) that exits from the anus is pure (tahir), and there is no need to wash the private parts or the clothes because of it. Similarly, there is no need to perform ablution (wudu') for every prayer because of it. It is advised to disregard this matter.