CAIRO — For years Ebrahim Metwally Hegazy, an Egyptian human rights lawyer, documented the plight of Egyptians who vanished into the hidden recesses of his country’s powerful security apparatus.

Mr. Hegazy had a personal stake in the issue: one of his sons disappeared at an Islamist rally in central Cairo in 2013 and has never been found.

Then this week Mr. Hegazy himself suddenly disappeared without a trace and for a brief time joined the ranks of the people he has represented.

Human rights activists have frequently been harassed by the Egyptian authorities, and Human Rights Watch issued a report just last week that accused the police and security forces of torture and other abuses.