The 757, if I remember correctly does have a probe at the back of the engine as it uses EPR to set thrust and needs a probe in each end of the engine to measure the difference in inlet/outlet pressure. The CFM56 only has one small probe in the inlet as it uses N1 to set thrust, so the chances are a bag would have just wistled through and nobody was the wiser, especially as the bypass is 5:1 so a lot of air going around the core.
As with all these things, who really knows for sure but for pure airmanship, better go back to stand and have it checked out by an Engineer.If the engine developed a fault on takeoff on account of it, it would be hard to justify in court!
Al.