Is what I did — secretly searching the thief's room and taking the phone without informing him, based on the hadith "Whoever conceals [the fault of] a Muslim" — an error from a Sharia perspective?

The basic principle regarding espionage (tajassus) is that it is impermissible due to the harm it causes to others and the invasion of their privacy, as Allah Almighty says: "And do not spy," and as the Prophet, peace and blessings be upon him, said: "Do not spy and do not eavesdrop." However, spying is permissible if there is a strong suspicion that someone has taken the mobile phone, because it is your right to retrieve your phone, and because your search is less severe for the thief than a police search. It is also advisable for the thief to refrain from this act, as part of enjoining good and forbidding evil.