Isomorphic agents with different preferences: any suggestions?

Would you consider computer viruses as limited agents trying to appear as identical (superficially) as the unaltered system as possible?

Also note that the actual change between the original system and the altered system can be arbitrarily small though the change in behavior can be extremely large. Consider for example the Ken Thompson hack or the recent single gate security attack.

Farmers are nursing small pigs like their children, but later kill them and eat them. It may be unpredictable for pigs.

A spy who works like an ordinary person, but sometimes stole information.

I think you should make a distinction if the different behaviours comes from different circumstances or not.
If their environment is always the same, then I think the only to have what you ask is if the system has a hidden, very specific parameter, that says "when X and Y and Z happens, zig instead of zagging".
Otherwise, if the model is slightly chaotic, then an important alteration to the environment might provoke very different behaviour.

For the first type of agent, think of two Markov chains almost identical, only one has a very improbable arc to a stable subnet that doesn't exists in the other chain.
For the second type, think of two similar strange attractors, that have different behaviours away from the stable parameters. They will be approximately identical in the same zone and be very different away from that zone.

Agents based on lookup tables.