When a Discharge Lamp is brought into operation (initial Arc becomes a Plasma), a charge is placed on the Lamp's "Envelope" (the outermost part of a Lamp).
This charge is the result of the internal ionization of materials, which rapidly threw things from a steady state (prior to passing the precursive initiating Arc), into a different state.
Normally, this charge will drain off the Lamp, and allow the full Plasma to be achieved without impeding the start process - or if the start process is completed, the charge levels become low enough to not be noticed from someone touching a Lamp bare-handed.
This is very common with "Low Output" and Low Power Fluorescent Lamps, being very close to metallic items.
Most HID Lamps fall into this category too, mainly because they have their Arc Tubes enclosed by separate envelopes, so the charge simply drains off through the socket connection, into a larger metallic body (also I have no wish to touch any HID Lamps in operation!!!).
On the other hand (OTOH), Fluorescent Lamps having High Output classifications (like F40 T12 HO - an 800 ma High Output Lamp, or F96 T12 VHO - a 1500 ma Very High Output Lamp), will leave a much higher charge on the Lamps, and these charges may not always completely drain off to metallic items in close proximity of the Lamps - resulting in "EEEOWWW" from the person grabbing a Lamp bare-handed.
The phenomina taking place here is not an actual (or required) "Path To Ground" for the charge on the Lamp to "find", but the "ability" for charges on the Lamp to flow to "something" having a different charge (could be higher or lower in charge state).
This is the old statement: "Difference Of Potential"
In the case of the Lamp zapping, one would not need to be connected to the Earth, or any other Ground Bonded Metallic structure/equipment - one simply needs to have a different state of charge, and there will be "A Difference In Potential" - thus allowing charges to transfer (in hopes to become "Neutral" or "Ground State").
Sadly, the unwilling victim has never been asked to nullify the Potential Difference!
Physics just assumes you are in full agreement, and never asks / says a thing!