The phrase red flag to a bull means that something will make someone angry, i.e., In this case, Mike is going to be angry if he finds out that Tom got promoted.
Context: "Don't mention Tom's promotion to Mike as Mike is going to feel pissed about it."
Hypothesis: Don't mention Tom 's promotion to Mike as it would be a red flag to a bull.
Based on the context, is the hypothesis true, false, or unknown?