Agentless Passive in Technical English

"The production database was accidentally dropped last night" is correct: when the agent is unknown or unimportant to the message, the passive voice can simply omit the "by" phrase entirely, keeping the sentence natural and focused on the affected object. Option A awkwardly states "by someone unknown", which is redundant since omitting the agent already implies it isn't known or isn't important. Option C uses the active-looking "dropped" without an auxiliary, making it read as if the database performed the action itself, and clumsily adds "by an unknown person" afterward. Option D produces an ungrammatical structure with "by unknown" used as if it were a complete phrase, which is not standard English.

"The pull request was merged into the main branch" is correct: since the reviewer's identity is either irrelevant to the report or already clear from surrounding context, the agentless passive keeps the focus on the action and its result. Option A restates the obvious agent ("by the reviewer who approved it"), which adds redundant, unnecessary information the passive is specifically designed to omit. Option C incorrectly implies the pull request acted autonomously ("by itself"), which misrepresents the process, since a pull request cannot merge itself. Option D leaves an ungrammatical, unresolved question word "whom" inside a statement, which only makes sense as an actual question.

"...was patched by the security team, not by the vendor as originally assumed" is correct: here the agent is the key new or contrastive information (correcting an earlier assumption), so it must be stated explicitly with "by", and the sentence maintains standard passive structure with the auxiliary "was" plus past participle. Option A omits the agent entirely, losing the important contrastive fact the sentence is built to convey. Option C drops the auxiliary "was", leaving an ungrammatical fragment that reads as a noun phrase rather than a full passive clause. Option D has awkward, non-standard punctuation and drops the required "by" before "vendor", breaking the parallel structure of the contrast.

"Unit tests are run automatically before every deployment" is correct: in formal process documentation, the agentless passive is standard for describing routine, automated procedures where naming a specific performer would be unnatural or unnecessary. Option B illogically mixes an active subject "someone" with a stray "by the CI pipeline", creating a confusing double-agent structure. Option C adds an oddly vague and unnecessary agent, "by a person", when the whole point is that the process is automated and agentless. Option D tacks on the awkward, informal phrase "done by nobody in particular", which is redundant and stylistically inappropriate for a technical report.

"The root cause has not yet been identified" is correct: this agentless present perfect passive appropriately signals that an identification process is incomplete without needing to name who is responsible for it, which fits the tentative, ongoing nature of a postmortem investigation. Option B adds a vague, unnecessary agent phrase ("by whoever is investigating it") that adds no useful information and undermines the concise, agentless style postmortems favor. Option C switches to an awkward active structure with the redundant phrase "by unknown means", which doesn't logically modify "identified". Option D omits the auxiliary "has", leaving an incomplete, ungrammatical passive construction.