i found a higher quality version, not sure of the original source, and its definitely AI:

regardless though, i did look into if this is possible. no matter if the connector on the vehicle is NACS/SAE J3400 (the "tesla" connector") or SAE J1772 (CCS 1), for AC charging—which is the situation here—it will be the same (note that this will not exactly apply to vehicles using CHAdeMO, GB/T, etc.). i went with this document that briefly outlines NACS. this is the proper standard, but i wasnt able to find a copy gratis with my two minutes of searching. this should still apply to other vehicles with a CCS 1/J1172 connector, at least for AC charging!

now, since we are charging with alternating current, we have only five pins to deal with, seen here for NACS (for CCS 1, just ignore the two super giant pins, if present. otherwise it applies to CCS 1 too!):

the big two (HV + and HV -) are for supplying power, no matter the flavor of AC or DC at hand. with CCS, DC charging goes through the two big pins at the bottom, AC charging works the same way though. the ground pin is present for low voltage electronics to actually have, well, a proper reference to ground! (this is your zero volt pin).

finally, we have the control pilot, and the proximity pilot. the former is a digital interface for the vehicle and charger to communicate in depth about charging speed, battery status, etc. the latter however, which is of the greatest interest to us, handles the physical connection itself. this is where the name comes from: it handles when a charging connector is in proximity (plugged in) to the vehicle!

when measuring between the ground pin and the proximity pilot pin, if the vehicle is ready to accept a charging cable but none is plugged in, there will be five volts. if there is something compatible plugged in, resistors within the connector will pull the voltage down lower (to 1.5V), indicating that something is, well, plugged in!

here is the part of the charging connection schematic that is relevant to us:

R1 and R2 are just resistors (150Ω and 330Ω, respectively), and S1 is a (optional to implement) normally-closed switch—likely a button on the charger plug handle—that brings the voltage slightly higher (to 2.8V), to signal to stop charging and if possible, unlock the connector (if it is locking).

now, the resistors themselves are located inside the plug, and not the charger/power control unit. after all, if the vehicle manages to somehow move away, and the cable snaps off said unit, the vehicle can still be aware of if something is plugged in!

of course, if you want to defeat this, it is very simple: tape over the proximity pilot pin! as far as the charge port door goes? from forum posts i've seen, the vehicle will still drive, even if it's a little whiney.

"but wait!" you say, "the vehicle can latch the connector into itself. can't it just detect it that way?"

this would certainly vary by vehicle. for teslas, it likely can't (or if it could, the engineers didn't bother to). seen in this youtube video, the charge door slams into the object left in the charge port, indicating the vehicle probably doesn't look at this.

i also looked up the electrical schematics provided by tesla here (SVG):

the block on the left is the vehicle-side charging port assembly, the right is the actual vehicle computer. the three pins at the bottom handle the connector latch: X312-1 and X312-3, which power the motors, and X312-2, a switch/button in the port which is presumably what the vehicle uses to know it latched onto something successfully. remember, this is for the latch the vehicle extends—not the one that is simply mechanical and operated by the driver—and most likely does not provide enough information to tell if something is plugged in.

overall, if we are talking about teslas, your point does hold some merit; even if you snipped the charging plug entirely off a cable, the vehicle can still (if the plug is up to standards) detect it if it's plugged in. however, something like this would not be unrealistic, even if the vehicle didn't actually charge.

of course, this may not be the case for other vehicles! but that is beyond the scope here :)