Knowing the exact number of postal votes before any vote is cast is easy. Knowing who those votes are for is impossible.
Candidates, agents and parties all can apply for a copy of the full register. This copy varies slightly from the one you can examine/check as there's more detail on it. They are also supplied with a copy of postal voters, which is in the same format as the full register supplied to them. Sit down with a pencil and street map and you can soon see areas of high postal votes. And they'll have previous years records to compare it with.
All this is why postal votes are getting harder to get. Why you now have to apply more frequently than before.
Maybe nothing was done because no complaint was made by anyone else standing or if someone who knew before the count decided not to question it. It needs that complaint making, that question being asked in order for anything to be done. Otherwise it's just hearsay/rumours, and they can't investigate either of those.