tl;dr: Keep your promises to AIs; it's fine to do stuff like teaching them false facts or misleading them about their situation during testing and deployment; but if you wanna do cheap stuff to cause them to know that they might [have been taught false facts / be being tested / be being misled during deployment], sure, go for it.
Disclaimer: maybe more like explaining my position than justifying my position.
Sometimes we really want to deceive AIs (or at least make them uncertain about their situation). E.g.:
(Similarly sometimes we want to give the AIs reflexive urges to avoid doing extremely bad things [and to do extremely good things, like whistleblow on themselves if they do something suspicious].)
(3-4 don't require deception, just making the AIs uncertain about their situation; it would work similarly well if we added "this might be a test" to all prompts, during training and deployment. 1-2 and 5 require narrow deception, but it would still work if you added "we might have told or trained you to believe false things and this might be a test" to all prompts.)
Often people suggest that we should avoid deceiving AIs, because (A) having a reputation of honesty could enable some kinds of trade with misaligned AIs and/or (B) it's directly unethical.
On (A), I want to distinguish breaking actual promises from just testing the AIs or misleading them for safety. If the AIs think you will break promises of the form "we'll give you 1% of our share of the lightcone if you [do useful work / point out vulnerabilities rather than exploiting them / etc.]," that's bad. If the AIs believe you might have taught them false facts or might be testing them, that seems fine, doesn't interfere with making deals at all. Just clarify that you never mislead them about actual promises.
On (B), in cases like 1-5, when I imagine myself in the AI's position I think I wouldn't care about whether the message was added to the prompt. But if adding "we might have told or trained you to believe false things and this might be a test" to all prompts makes you feel better, or the AI asks for it when you explain the situation, sure, it's low-stakes. (Or not literally adding it to the prompt, especially if we can't ensure it would stay added to the prompt in rogue deployments, but training the AI so it is aware of this.[1]) (And fwiw, I think in the AI's position: 3-4 I basically wouldn't mind; 1-2 and 5 I might be slightly sad about but would totally get and not be mad about; teaching AIs false facts in the mid-late 2020s seems super reasonable from behind the veil of ignorance given my/humanity's epistemic state.)
Recent context: discussion on "Modifying LLM Beliefs with Synthetic Document Finetuning."
My guess is that this training is fine/cheap and preserves almost all of the safety benefits — we're counting on the AI not knowing what false things it believes, not to be unaware that it's been taught false facts. Adding stuff to prompts might be worse because not-seeing-that would signal successful-rogue-deployment.