Open limp on button, should I be concerned?

[nightlyraver]

You should not be concerned - you call. You are getting huge odds of almost 4:1 on your call. Villian could easily have a second best hand. Perhaps he was sandbagging AA-QQ, maybe he has a 7 with a good kicker, maybe he has 76 or he rivered 2 pair, or he's just bluffing - who knows? Those are all reasonable probabilities. You only have to be right a 20% of the time to break even. I would guestimate that you are way better than 50/50 to win this hand so you make this easy call.