You can deduct theft losses that aren't reimbursable
only in the year you discover your
property was stolen. So if it was discovered in 2017, 2016 or 2015, you can claim it for that year on an amended return . The loss could produce an NOL. the NOL would need to be carried back. but any remaining NOL could be carried forward.
I agree with RJS. Nonbusiness Deductions for Form 1045 Schedule A line 6 exclude casualty and theft losses reported on schedule A. So in effect all casualty losses are treated as if from a trade or business and thus can produce an NOL.
