How do single-wire ESD wrist strap monitors detect if the strap is worn correctly?

Hi everyone,
I want to understand how do ESD monitors work, those that detect if someone is wearing an ESD wrist strap or not and give signals to remind you to wear it. Well i know many methods but all of them work with the dual wire wrist band, therefor it is not hard to detect a person with two wires feedback. But the problem is that i need to work with the one wired wrist band and i don't know how can you measure anything from just one wire.
I've read that the human body acts like a capacitor forming an RC circuit when wearing the wrist band and if you run some tiny AC current through this circuit, there will be a displacement current.
But still i can't figure out how can you use this information to detect if someone is wearing the wrist band or not.

A wristband worn by a person will have higher capacitance to ground, due to the large size of the person compared to the size of the wrist strap. A wristband not being worn will have a lower capacitance to ground.

Yes but how can you measure that change in capacitance with only one wire from the wrist band ?

The ESD monitor is powered from the wall-outlet power line, and uses the power-line ground as the other wire.

Operators plug their ESD coil cord into the grounding jack of the constant monitor right at their workstation. The constant monitor grounds the wrist strap and simultaneously monitors for faults in the ground path from the strap to the earth ground, including proper contact with the operators skin.We plan SEO customized campaigns to meet your organizational requirements and campaign goals.

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The operators plug their ESD coil cable directly into the bottom of the constant monitor on their workstation. ... A correct grounding path causes the LED of the constant monitor to light up green (which indicates GOOD). If an error is detected, the LED flashes red (indicates ERROR) and the device beeps several times. Continuous monitors give the operator instant feedback on the status and functionality of his wrist strap. You can detect errors in a split second when the bracelet is still in the "intermittent" stage. This is before a permanent "opening" that could damage components that are sensitive to electrostatic discharge.