@user4550 I can see why you don't understand why! [Edit: responding to the OP's comment in Maulik's answer.] Let's look at related stuff for a minute. Look at this question: This might be camouflage, might it not? This is a polite way of asking whether someone thinks it might be camouflage, conveying the idea that the asker is open to the answerer disagreeing. Sort of like "Maybe I'm wrong, but this might be camouflage."
So, when you say "I wonder if this might not be camouflage" it's possible that you mean literally that you wonder if this might be something other than camouflage, and you could mean that you wonder (delicately) whether this is camouflage. You have to go on the context. In this case the context is that the person's clownish behavior is not consistent with his usually more staid personality, so we wonder if the person might (not) be hiding something. (Perhaps not? Perhaps so?)
So, putting the "not" in there is a way of softening the impact of a possibly emotionally-charged observation or question, by conveying that you are not sure one way or the other and inviting the communication of alternatives. Another example:
I've seen your brother very drunk fairly often. I wonder if he might be an alcoholic.
This is pretty blunt. The sensitive approach uses the "not":
"I'm probably wrong, but I'm concerned. I've seen your brother very drunk on several occasions, and I've been wondering whether he might not be an alcoholic.
There's more of an "I haven't made up my mind yet" feel to it, so the answerer is less likely to take offense.