There are any number of ways to bypass the security perimeters of big estates. Most people who live there don't really surround themselves with tight security because it's intrusive (often not convenient or fun for them). 

If a consultant tells them to tighten up, they bridle/whine and the consultant backs off. 

Most people who live on big estates play a numbers game - figuring correctly that the odds on any given day of being targeted by a dangerous penetration of their grounds is slim, so they learn to forget about it, and never really tighten up their P and P's.

Distance from the urban centers is probably the best deterrent to incursions (the more-isolated, the more-secure). But even then people find ways to breech perimeter barriers/check-points. 

I have worked at some of the most exclusive gated estates/ communities in Mexifornia and have accurate knowledge of many others. Incursions by random (mostly-harmless) nuts was fairly common, even though billionaires, celebrities and political figures lived there.  

Former Beatle George Harrison was a tragic case-in-point, when a lunatic jumped the tall wall around his house, broke into the house and stabbed him repeatedly. He survived the attack initially but later died from lung failure, no-doubt aggravated by the lacerations from the stabbing. 

People who are REALLY serious about security simply choose to live out in the middle of nowhere and only commute into the city to work, where they can live (provisionally) in truly secure locations and can enjoy fairly tight (top-echelon corporate or LEO-run) security.