"Noor returned home himself this morning," DCP (Detection) Dhananjay Kulkarni said.
Now the Anti-Terrorism Squad (ATS) is probing reports that he, along with three others - Wajid Shaikh, Mohsin Sayyed and Ayaz Sultan - had left home to join the terror outfit.
Noor was the fourth youth who disappeared from Malwani area, a western suburb of Mumbai, in mid-December to reportedly to join the IS.
The whereabouts of two other youths (Sultan and Mohsin) are still not known, police said.
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Sultan went missing on October 30 and Wajid and Mohsin since December 16, a police official had said.
According to police, Sultan left home on October 30 after telling his parents that he had to go to Pune in connection with a job offer he got from a Kuwait-based firm.
Mohsin left home on December 16, saying he was going to attend a friend's wedding. Wajid also left home the same day saying he had to get the name on his Aadhar card corrected, police had said.