I fish there a lot during the day and have never seen any trouble. I have also spoken many times to lads fishing longer sessions and they have never had problems.
Having said that, I have also heard the same rumours, and I wonder who would gain by making it up. The lakes are spread over a large area, so it may well be that staying in the "Hot" areas with a few others around would keep you safe and if you venture of somewhere away from the beaten track you could find yourself in trouble. It could also be possible that people are drunk/noisy at night and upset the locals.
Personally, I wouldn't feel comfortable there on my own at night, so my rare overnighters are spent elsewhere. Partly just from the "heeby jeebies" and partly from things I have seen that hinted towards a base to the rumours. I don't really want to elaborate as it is just a bad vibe more than direct experience, and I don't want to be unfair to people who may well be entirely innocent.
I would suggest you take a look around. Perhaps fish a day (the pike are very good sport in winter) and talk to the others there, or just take a look around. Look at the complex on the map and check out the accessible areas of all the lakes, not just the main lakes off the causeway. Perhaps you will see what I saw and know exactly what I mean