The point is the transparent recycling bags also contained personal details of the Liberal Democrat's constituents. Whether it was potentially mundane or not is irrelevant, it breaches data protection. So your speculation about what may or may not have happened does not carry things forward.
OK, I accept the reasonableness of the factual position as you set it out. Technically there is no ruling and there is room for doubt and as a result Cable could be in the clear.
I formed my opinion from two quotes, one from a spokeswoman from the ICO who said "This is the second example in recent weeks of an MP or their staff allegedly mishandling constituency paperwork and again highlights the need to ensure that personal details are kept secure," I acknowledge the word "allegedly"
Secondly from Vince Cable himself when he is quoted as saying "Nonetheless, some correspondence which should have been protected was placed in bags for recycling outside the office.
"I apologise unreservedly to all my constituents for what has clearly been an unacceptable breach of their privacy."
My recycling sacks are transparent and I don't think that the local council is stupid for having transparent bags. I don't put discarded correspondence in them un-shredded and neither should Vince Cable's staff. I haven't heard that any disciplinary action has been taken against the member of staff.