Methods for maximising a sense of isolation for a PEC ...

[1] Build the PEC in the remotest corner of the map
[2] Provide access only via a dirt track
[3] Place the Pensioner Extraction Centre road sign at the very start of the track
[4] Use TM:PE to prevent all Private Vehicles from using the track (applying the restriction to the first and last segments is sufficient - it'll still allow employees to reach the PEC and its Services)
[5] Use TM:PE to remove all Parking on the track, and instead provide a Car Park for employee vehicles
[6] Use TM:PE to set speed limit to 'unlimited', so that Service Vehicles can quickly access and leave the PEC despite its remote location

NOTE: The issues below only arise with a completely isolated PEC - they can all be fixed by connecting your PEC to the rest of your city

If your PEC is not connected to the rest of the city, the situation below will eventually occur, and can afflict any Service that's also being used by the PEC (Ambulances / Fire Engines / Police / Hearses / Garbage Trucks)

Examples
[1] There is a fire in your city, and - even though Fire Engines are available - none are despatched to attend the fire. Similarly ...
[2] A house next door to a Fire Station is on fire, yet - instead of summoning a Fire Engine from the station next door - a Fire Engine is despatched from the other side of the city

Both these scenarios are the product of the way CS code works:
[1] All available Fire Engines in a city are stored in a 'stack'
[2] When there's a fire, whichever Fire Engine is at the top of the stack is the one that gets despatched, period

That's why when you build a new Fire Station, all the fires in a city are suddenly being attended by engines from that new Station ... because those engines have all been added to the top of the stack

How does a PEC impact those scenarios ...?
Well, if the next available Fire Engine in the stack happens to be one located at a PEC that's completely isolated from the rest of the city, then it can never reach its destination, so the situation never gets resolved

Simply turning off a PEC won't work, as Garbage and Crime etc will still accumulate. You can however try turning off the PEC's Services, as that will remove its Service vehicles from the stack

This issue won't affect any other part of a city - only completely isolated PECs ...
[1] Hearses and Garbage Trucks normally make multiple pick-ups, and only return to base when they're full
[2] Trouble is, even a fully populated PEC won't produce enough corpses and garbage to fill even one vehicle
[3] So - because the Hearse / Garbage Truck can't find any more loads - instead of returning to base, it simply despawns
[4] That's a known CS bug and graphical glitch. It's not a big deal though, as the partially complete loads are still delivered to base

This may actually be considered a desirable behaviour, as it's one way of removing Seniors from your city! The issue arises when Service vehicles fail to keep on top of an ever-increasing amount of Garbage / Crime / sick Seniors