I saw a neighbor taking something out of another neighbors mailbox and am not quite sure what to do about it. At first I thought maybe they'd left something for him in there, then I remembered they'd been gone for a couple of days so that wouldn't be likely. I don't know either of the homeowners very well and I'm thinking that may be my excuse for not getting involved.
There are a lot of desperate people out of work these days, possibly desperate enough to steal checks or banking information from neighbors, so I think you need to speak up about what you saw. Once your neighbors return from their trip ask them if the other neighbor was supposed to take their mail to his house while they were gone. If they say yes then you can be relieved that you're not living next to a criminal. If they're shocked to hear that someone was messing in their mailbox then you know you did the right thing. Definitely say something as opening someone else's mailbox is a federal offense.