This could be done in one of two ways:
1. Hidden character trait - Any character (except the starting ones) that joins the colony could come with a hidden status (or bodypart, if they even can be hidden) making them a Mechanoid Sleeper Agent.
2. A random event that can turn any existing character into a Mechanoid Sleeper Agent on the spot. This is the less favourable option, since it could also affect the starting characters, but it'd probably be far easier to implement.
Now, the Mechanoid Sleeper Agent (henceforth called MSA) would be a humanlike machine (think terminator infiltrators) that will reveal itself at a least opportune time (for your colony) to aid their mechanoid overlords against you (so during a sufficiently big mechanoid raid), turning against its former comrades.
In case you manage to successfully neutralize the MSA, its body will contain a whole slew of high-tier prosthetics and - on a rare occasion - a techprof subpersona core.
As a tangentially related idea, there could be an Empire Inquisitor event - one of your colonists caught an eye of the Stellarch's Inquisition, and must be put under investigation.
Even if the colonist is proven to be human, the Inquisitorial investigators - being known for their carnal methods of interrogation - will leave your colonist in a lasting pain and a resulting mood debuff, but will reimburse you for the trouble.
In case the colonist turns out to be MSA, you'll have a choice of either executing them (you get the loot mentioned in a previous paragraph) or give them away to the Empire for some royalty-related diplomatic favor. Or keep the colonist to disable their Hive Uplink (you get a heavily augumented supercolonist that's no longer MSA) and fight the Empire with all its wrath.
1. Hidden character trait - Any character (except the starting ones) that joins the colony could come with a hidden status (or bodypart, if they even can be hidden) making them a Mechanoid Sleeper Agent.
2. A random event that can turn any existing character into a Mechanoid Sleeper Agent on the spot. This is the less favourable option, since it could also affect the starting characters, but it'd probably be far easier to implement.
Now, the Mechanoid Sleeper Agent (henceforth called MSA) would be a humanlike machine (think terminator infiltrators) that will reveal itself at a least opportune time (for your colony) to aid their mechanoid overlords against you (so during a sufficiently big mechanoid raid), turning against its former comrades.
In case you manage to successfully neutralize the MSA, its body will contain a whole slew of high-tier prosthetics and - on a rare occasion - a techprof subpersona core.
As a tangentially related idea, there could be an Empire Inquisitor event - one of your colonists caught an eye of the Stellarch's Inquisition, and must be put under investigation.
Even if the colonist is proven to be human, the Inquisitorial investigators - being known for their carnal methods of interrogation - will leave your colonist in a lasting pain and a resulting mood debuff, but will reimburse you for the trouble.
In case the colonist turns out to be MSA, you'll have a choice of either executing them (you get the loot mentioned in a previous paragraph) or give them away to the Empire for some royalty-related diplomatic favor. Or keep the colonist to disable their Hive Uplink (you get a heavily augumented supercolonist that's no longer MSA) and fight the Empire with all its wrath.