It is possible only if the incoming power for some reason runs out to the light fixture first then comes back to the switch as a switch loop. Otherwise, there should not be active power in any of the lines out there if the switch cuts it off.

You might be on to something there. It should be pretty easy to determine whether the switch is improperly wired (that is, in series with the grounded conductor).

I'd also shut off all power to that circuit, disconnect everything at both ends of the buried cable, and check each cable conductor's resistance to ground with a good ohmmeter.

I've seen switches end up in a grounded conductor by accident. It can happen when an old house with knob and tube wiring is upgraded, either with a new panel or partial rewiring, and the K&T is left in place and connected to the newer panel or wiring.

With that old stuff, it's often tough or impossible to tell by looking which wire should be hot. If you don't check carefully, it's easy to wire it backwards.

I experienced this personally in an old house I owned. I was changing a light bulb and carelessly touched the base shell. I got a surprise, even though the switch was off.

Otherwise, do you have an LED "bulb" in the light? Is the switch controlling it a little unusual in some way? For example, could it be a timer or dimmer, or might it have an illuminated toggle?

ok, thanks, It's a regular toggle switch. So I guess I'll test voltage at the light while it's switched off first to see if it's wired backward. that would be easiest to correct. but if it's not that then some other voltage must be connected to a ground for there to be partial voltage at the light right? I expect to see a partial voltage since a see a partilly lit light. Is that potentially bad?

I ask again -- is the light an LED? Depending on design, LEDs will sometimes glow with tiny (leakage) currents.

If it's incandescent, that would be more surprising and more unsettling.

Phenomena like this can be tough to track down. It can be a combination of factors and you just have to keep looking, tracing, experimenting, and diagnosing until you spot what's going on. Think like an electron. :)
As I said above, I would probably start with isolating the light and its wiring, and making some resistance tests with a multimeter, looking for places that should read infinite resistance (or well into the megohms) but instead read in the tens, hundreds, or thousands of ohms.

This is an intriguing puzzle! Please let us know what you find.

Sorry Yes it's LED bulbs now, but it was incandesent before and the problem existed. Will update if I figure anything out, thanks!