Diagnosing appliance that shocked me

Question

I have a portable air compressor (Master Airbrush MAS TC-40T) I use frequently, usually for airbrush painting. Normally the compressor sits on a counter, and I do not touch it while it's running because I have no need to. However, yesterday when I touched the compressor while it was running, it shocked me several times.
When I got shocked, I was using the compressor to blow dust out of a ventilation fan in the bathroom ceiling. The compressor (which has a three-prong grounded power cord) was plugged into the bathroom's GFCI grounded outlet. I had the compressor balanced on top of a ladder and was leaning past it to point the spray nozzle at the ventilation fan.
Every few seconds I'd accidentally brush against the compressor's casing and get a shock. The first few shocks were very brief and I chalked them up to static electricity. However, one time when my finger touched the casing, I received a sustained shock that lasted a half-second or so until I pulled my finger away. At this point I disconnected the compressor and stopped using it.
Now I've done everything I know of to try to diagnose the problem and haven't found anything wrong:

The Test/Reset buttons on the GFCI outlet are working correctly
When I put the multimeter into AC Volts mode, insert the black probe into the outlet's ground slot and the red probe into the outlet's hot slot, the multimeter shows a reading of ~118V, which I think means the outlet is grounded correctly (I'm in the US with 120V power).
When I put the multimeter into Resistance mode and touch one probe to the casing and the other to the ground prong on the power cable, the multimeter shows a very low resistance reading, which I think means that the compressor is internally grounded correctly. I've tried this with the compressor in several orientations in case the internal grounding wire is loose, but the readings are always about the same.

I can't think of anything else that could be wrong that I should be testing for. Have I tested everything that I should test correctly? Should I chalk the shocks up to static electricity?

Kevin · Accepted Answer

Thanks to everyone and particularly to ThreePhaseEel for his suggestion that helped me diagnose the issue.
My non-contact voltage detector picks up current within about 18 inches of the fan housing, even while the fan is turned off. The other two bathrooms have the same model fan assembly installed, but the voltage detector doesn't read anything from just beneath their housings, even when they are turned on!
Clearly something is not wired correctly in the bathroom where I got shocked, and the shock which I received when I touched the air compressor must have originated from the fan assembly/housing, which is apparently hot all the time.
I didn't get shocked when I removed the cover from the fan assembly, or when I touched the inside of the housing while inspecting it, but maybe that's because I was standing on a fiberglass ladder. My guess is that I got a shock when my body formed a circuit between the electrified housing and the air compressor (which was plugged into the wall and thus able to continue the circuit), and I just happened to be feeling the shocks on the air compressor side even though the current was coming from the fan housing.
Update:
The cables on my multimeter are not long enough to reach from the fan housing to a safe grounding source, but my uncle brought over a wired voltage tester with long cables. We put one probe into the ground port of the bathroom outlet, and touched the other probe to the fan housing. The tester immediately beeped, verifying that the housing is hot.
Update 2: Final diagnosis of the disastrous wiring

The fan (which is a 1500W combo unit with a heater and dome light) was on the same 15-amp circuit as all of the ceiling lights in that half of the house. I don't know how it never tripped the circuit breaker.
The vanity lights were on their own 15-amp circuit. Obviously whoever originally did the wiring mixed up the two, as the fan should have been on its own circuit.
The fan housing has a grounding wire, but the grounding wire was not connected on either end.
The fan housing was hot because the wiring to the vanity lights was frayed at the junction box, which is attached to the same metal framework as the fan housing.

user69795 · Answer

If the outlet safety ground is not really grounded, but is open or perhaps connected to neutral, this could explain what you described.  Check the outlet ground to water pipe resistance. This should be low.
If the outlet is correctly grounded, then since the compressor case is connected to safety ground ( as the ohmmeter verified), this means your body was connected to hot. You must have been touching something else that was leaking current through you to ground.

Diagnosing appliance that shocked me

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