"Karim Asaad is out and on his way home," said Khaled Elbalshy, president of the syndicate of Egyptian journalists, in a Facebook post.
His fact-checking service Matsada2sh confirmed late Sunday that Asaad was back home.
Asaad was apprehended at his home in Greater Cairo early Saturday after Matsada2sh had published articles concerning the smuggling case, the independent website said in a statement earlier on Sunday.
"Our colleague, journalist Karim Asaad ... was arrested after security forces dressed in civilian clothes stormed his home" at 1:00 am on Saturday (2200 GMT on Friday), said Matsada2sh.
"They physically assaulted his wife, threatened their young child, raided the apartment, and then led him away, forcefully disappeared, to an undisclosed location."
The journalist syndicate had earlier called on authorities to "free" Asaad.
According to the union, the Arab world's most populous nation keeps 23 journalists behind bars.
There was no official comment concerning Asaad's detention.
According to the Matsada2sh statement, "before his arrest, the only questions the assailants asked our colleague were related to our breaking coverage of the Zambia-Egypt plane story."
The southern African country's Drug Enforcement Commission on Tuesday announced its officers had seized "a chartered aircraft carrying dangerous goods" at Lusaka airport.
The plane was carrying nearly $5.7 million as well as pistols, ammunition and 127 kilos (280 pounds) of "suspected gold," according to the statement.
The Zambian authorities have arrested 10 suspects, nine of them foreigners, it said.
Lawyers' documents in Lusaka note that at least five Egyptians have been detained.
Independent Egyptian journalists have published over social media documents purportedly from the Zambian probe that name Egyptian suspects in the case, including army and police officers.
Egyptian state media claimed the aircraft in question was privately owned and that it only transited through Cairo.
"So far, the only reaction from the government was to arrest ... one of the only sources of information on this topic," argued activist Lobna Darwish of the Egyptian Initiative for Personal Rights in a social media post.
Matsda2sh, whose Arabic name translates into "Don't believe," was founded in London in 2018 by the late Egyptian journalist Mohamed Aboul Gheit.
The website said its staff and social media pages have been "subjected to a coordinated attack" since the plane report.
Two posts "related to our coverage" of the case "in which senior Egyptian government officials were involved" were deleted from Facebook, it added.
"We hold the Egyptian government responsible for the safety and well-being of our team."
Media watchdog Reporters Without Borders ranks Egypt 166th out of 180 countries in its World Press Freedom Index.
Cairo has faced frequent criticism of its human rights record, from incarceration of dissidents and freedom of expression to LGBTQ and women's rights.
On Sunday opposition activist Hisham Kassem was detained a day after being pictured alongside Ahmed Douma, a leading figure in Egypt's 2011 uprising who had just been freed after a presidential pardon.
Summoned for questioning following a defamation complaint filed by a former minister, Kassem announced on X, formerly Twitter, that he had refused to pay bail of 5,000 Egyptian pounds ($150).
On Saturday night, Kassem had also posted about Asaad's arrest, writing: "Even if you arrest half the country, we will still find out about the thefts and embezzlement of the people's money."