I record a ton of opera, though only a few times from an audience seat in stealth mode.
When I am in the audience and doing stealth, I have been happiest with a pair of Line Audio Omni1 omni mics spaced 14" (rigged them so that the caps stick out of the top of my backpack, which was on my lap). Not a standard setup around here. But, I don't know that I have ever seen serious security at an opera, except maybe Hollywood Bowl (which is not really an opera venue). You can get away with things that you could not do at a rock concert, and the audience is more cooperative too in terms of making noise.
But, if I were to make a dedicated stealth opera setup to record in the audience, I would use a pair of DPA 4060 (super small excellent omnis. More noise floor than big mics, but you don't care because the audience will already be at that noise floor). I would have preferred these to my Line Audio setup if I had them handy at the time -- way more stealth and they sound great.
Used ones are easy to buy since every theater and opera in the world has a stack of them. People here love them. Someone might say you want something with an HF bump if you're in the audience, but I don't necessarily agree -- doesn't sound right on opera to my ears or the ears of the opera singers I work with.