Yes. P2 finding this out would harm him, and couldn't possibly benefit anyone else, so if searching would lead him to believe the cube doesn't exist, it would be ethically better if he didn't search. But the harm to P2 is a result of his knowledge, not the mere fact of the cube's inexistence. Likewise, P1 should investigate assuming he would find the cube. The reason for this difference is that investigating would have a different effect on the mental states of P1 than it would on the mental states of P2. If the cube in U1 can't be found by P1, than the asymmetry is gone, and neither should investigate.
Very well, I repeat the advise I gave you above.
If you truly believe this proposition, as opposed to merely belief in belief, you shown stop reading LessWrong right now. If you keep reading LessWrong, you are likely to get better at rationality, and in particular at telling whether something is true or false, which [is likely to result in you discovering that a lot of gold cubes don't exist].
I figure morality as a topic is popular enough and important enough and related-to-rationality enough to deserve its own thread.
Questions, comments, rants, links, whatever are all welcome. If you're like me you've probably been aching to share your ten paragraph take on meta-ethics or whatever for about three uncountable eons now. Here's your chance.
I recommend reading Wikipedia's article on meta-ethics before jumping into the fray, if only to get familiar with the standard terminology. The standard terminology is often abused. This makes some people sad. Please don't make those people sad.