Looks like a standard probe bet from MP2. Probably a weak ace, but maybe a ten. If he flopped 2 pair or a set I would think he would most likely go for the check raise here as he would most likely expect you to make a continuation bet. I would raise here and solidly define your hand strength in relation to the others. I would raise somewhere from 600-800, and see if anyone plays with you. Let MP2 know he will have to commit all of his chips to see another card.