The laws about the signs here in Michigan only have effect if an employee asks you to leave. don't quote me on this, but as far as my knowledge goes you can carry into a place that is posted. If you are asked to leave then leave or you will then be trespassing, and they can then get you in trouble with the police.
Keep it concealed and you should be ok.
Oh and one more thing if it is a store that is posted I will either go in and explain to management how they no longer will be getting my hard earned money, and also send their headquarters an email regarding their "victim enriched zone"... sometimes they will take the sign down when enough people complain. Like Krogers in Ohio.