Mejia is in the B Block showers. The achievement is unlocked if Adam finds him without raising an alarm. Adam need not have advance knowledge of where Mejia is. If he stumbles into the showers without raising an alarm, the achievement will unlock. There are several clues, though, that provide story continuity. If you undertake the optional mission and deliver the Altered Biocell to Red Shoes, Red Shoes will inform you that Mejia is in the showers. The following guide provides information on how to get to Red Shoes if you need that location as a point of reference for this guide:Solution for A Just Cause In Deus Ex: Mankind Divided
From Red Shoes, it is very easy to get to the showers undetected. There is a secret passage in Room B204 to Adam's right as he reenters the cell block. The secret passage exits into a room next to the showers. To avoid setting off an alarm, Adam can cross the floor to the other side entering a laundry room through an open door. Closing the door for privacy, Adam climbs into a vent that leads to a vacant passageway to the showers. The achievement is unlocked as soon as Adam enters the laundry room assuming no alarms have been set off.
The below video illustrates the above route with an extra wrinkle. To find a clue relevant (but not essential) for the Objection!
achievement, Adam takes a small but risky detour. You may not wish to take this detour. It makes unlocking this achievement more difficult, it is not an essential requirement for the Objection!
achievement, and the item (a hacking device) can be picked up a little bit later in the game (but requires a larger detour). The video is provided for those who want to be thorough in their pursuit of information pertaining to Wilburg's murder. Nevertheless, you might prefer going directly to the laundry room without going out of your way, risking exposure, to get the hacking device: