It's apparently pretty obvious, too.
Tʜᴇre's a ʟᴏᴛ ʏᴏᴜ cᴀɴ mɪss ɪɴ Deᴀᴛhloop. It's spʀᴇᴀᴅ aᴄʀᴏss ꜰᴏᴜʀ regiᴏɴs, ᴇᴀᴄʜ wɪᴛh ꜰᴏᴜʀ tiᴍᴇ ᴘᴇʀɪᴏᴅs, ᴀɴd ɢɪᴠᴇn tʜᴇ reᴀᴄᴛivɪᴛy ʙᴇtᴡᴇen tʜᴇse regiᴏɴs ᴀɴd tiᴍᴇ ᴘᴇʀɪᴏᴅs, ɪᴛ's ᴘᴏssɪʙʟᴇ ᴛᴏ ᴄᴏᴍᴘʟᴇᴛᴇ a pʟᴀʏᴛʜʀᴏᴜɢʜ wɪᴛhᴏᴜᴛ dɪsᴄᴏᴠᴇʀɪɴg ʜᴇaps ᴏꜰ stuff. It's ʙᴇen nᴇᴀʀly a ᴍᴏɴᴛʜ sɪɴce Arkᴀɴe's lᴀᴛest releᴀse ᴛʜᴏᴜɢʜ, ᴀɴd ɪᴛ's fᴀɪʀ ᴛᴏ sᴀʏ thᴀᴛ tʜᴇ ɪɴternet's ᴄᴏʟʟᴇᴄᴛive efꜰᴏʀts ʜᴀᴠᴇ probably unᴄᴏᴠᴇʀed ᴇᴠᴇʀythɪɴg. Right?
Apparently not, according to Arkane level designer Julien Eveillé. "I know for a fact the Internet didn't find everything hidden in Deathloop so far," he tweeted. "There's still one secret thing I want someone to discover."
According to Eveillé's tweets, his work was predominantly on the Updaam region: both the noon and afternoon variants. So you'd probably expect that secret to be hidden there, but it's not that simple. In answer to a Twitter user querying the nature (and speculated insignificance) of the secret, Eveillé had this to say: "It's a way bigger consequence to a way bigger event that can only happen if you made it happen. Also the cause and the consequence aren't on the same map!"
In other words: don't just hit Updaam during noon or afternoon for the hunt, this is a tumbling dominoes kind of affair. And yet, it's not even that well hidden. "It's actually very 'on the nose' but is relying on a series of not that obvious odds," Eveillé tweeted.
In case you missed it, Deathloop is brilliant, according to our review.