You'd be surprised how easy it is.
I stood as a candidate in 2003 and I was shocked at how easily this information becomes available. With more intensive use of IT, a truly secret ballot will become impossible.
The staff in the polling stations, most of whom knew me, were not hiding the fact that the papers are not secure.
The facility allegedly exists to check for fraud, but actually opens the door for it.
To return, briefly, to Military Aviation, compare this to teaching Spin Recovery. The CAA no longer, for some years now, mandate such training as it was felt that more people died from learning how to handle it, than were at risk from it happening. My opinion is that the deadly spin happens when recovery would not help as there is insufficient height anyway; forced landing and stretching the glide being the popular cause.
So, we have a system where a Voter ID Number is allocated to each person entitled to vote. The polling stations have a list for each ward, showing everyone who can vote there. The authority to vote, is demonstrated by the possession of a Polling Card, bearing the Voter's ID number and full postal address.
Your vote is registered on a sheet of paper, the unique number from this piece of paper is written next to your name and number, on the list of voters.
This paper, which identifies You & Your Vote, is then stored, along with the sheets where your number (and name & address) is ticked off as having attended to vote.
So, it's an easy task to run down the list of registered voters and see all the blanks where people did not vote. Those people (the disillusioned and undecided) are worth targeting for the next campaign.
To go a step further, you cross reference the sheets with the numbered voting papers, and see what vote is recorded on each paper, which links back to a name and address.
Now comes the conspiracy theory part:
The paperwork is stored in the "care" of the local authority. The body who employ most of the invigilators.
The "non political" council officers, who depend upon the elected Members to choose them for high office, are of course above any kind of looking at this stuff, or allowing copies to be made. It says here.
The way in which some council staff cosy up to long term political office bearers makes me wonder. The whole system stinks, because of the hypocrisy.
The very first vote taken in the new Scottish Parliament was (taken in secret) to decide if they could have Secret Ballots.
That was hardly a step towards open democracy.