Melaye himself revealed this via Twitter on Monday morning, saying he was on his way to Morocco for an official assignment when he was arrested.

Senator Dino Melaye@dino_melaye
I HAVE JUST BEEN ARRESTED AT THE INTERNATIONAL WING OF THE NNAMDI AZIKIWE AIRPORT ON MY WAY TO MOROCCO FOR AN OFFICIAL ENGAGEMENT SPONSORED BY THE FEDERAL GOVT AFTER CHECKING IN.

He did not say which law enforcement agency (Police, DSS, etc) that arrested him, but when contacted, Anjuguri Manzah, spokesman of the Federal Capital Territory (FCT) Police Command, said he was not aware of the development.

https://www.leadingreporters.com/news/dino-melaye-arrested-at-abuja-airport/