The Search Spell is a type of Null Magic allowed the user to search and identify anything as long as what they are searching for matches the criteria of whatever the user has in mind.
In a way, the Search Spell is like using an Internet Search Engine to look for specific items using a keyword. For example, when Touya set the Search Spell to look for poison, he only found it on the glass of the King of Belfast's dining hall.
The main limitations of the Search Spell are its short range and the specifics of the keyword that Touya uses. If he is not specific in his search, the Spell may pick up things that are only remotely related to what he is looking for or not pick up anything at all.
Touya has been shown to overcome this limitation by using the map application on his smartphone.