This is an odd topic because I’m guessing there will be both people who had no idea about this and also those who have an exaggerated idea about this.
For those who are unaware, men in chastity will occasionally leak pre-cum, sometimes enough to where it will dribble out of their cage. The frequency of this directly relates to how often the man gets aroused. Even when chastity devices are involved, the penis will still attempt to get as erect as possible when aroused, although most devices limit this to a partially erect state. The penis will then react to the pressure against the cage, treating as stimulation and producing pre-cum as a lubricant.
Some of the beliefs about the frequency and amount of leakage that occurs often correlates quite heavily to how much time the wearer spends in chastity. For someone who stays locked up the majority of the time is more likely to feel like there isn’t a lot of leakage going on. e.g. 1-2 hours a day out of 24 is relatively infrequent.
For someone who is locked up only at certain times, frequently in an environment when dominance is present, they are more likely to spend a much larger percentage of locked time attempting an erection and are more likely to feel like leakage is a constant and continuous issue. Situations where a man may lock himself in chastity and specifically fantasize about arousing things are also more prone to excessive leakage.
So yes, leakage is real. As for how much leakage occurs, that heavily depends upon how chastity is being used. There will likely be zero leakage if a man doesn’t get aroused during that time frame. If he is locked up and continuously tormented in a sexual way, he’s more likely to drip like a leaky faucet.