Yeah, I'm thinking your problem may be originating from that chimney also. It's not the sheet bowing in the middle, it's the joints racking.
Seems like an awfully old roof to have two layers AND osb under it. Then again, you do live on the rusty side of the state.
THat thought crossed my mind too - we have had a lot of wind with rain over the last few weeks and that might have been enough to propel moisture under the chimney flashing. Not sure how old the roof is, but I do think it is not as old as it looks.
I am glad to get all the input - thanks to everyone for that. I am going to keep my eye on it and I hopefully get to the bottom of it in the spring.
That chimney roof flashing looks in rough shape. I bet you have water getting in under the shingles due to a roof leak around the chimney flashing area. A driving windswept rain could easily push water up underneath that flashing.
As others are saying I believe the joints are racking. I also believe plywood is the better choice on a roof.