"Just open the oil filter and see if it drained"

[Jamos]

... opened it and oil went EVERYWHERE! Derp.

But I really didn't expect it to be so full!? Was it just the filter holding on to oil, or is it pressurized somehow or... wow how the heck did that happen? Oil didn't really go EVERYWHERE but it sure as heck started flowing out, so I quickly closed it and cleaned up happy to see that the oil filter housing check valve thingy was still functioning as expected. Or so I believe.

Engine hadn't been on in a few weeks.

[Timm]

Check to see if the bottom flange of the filter is still in the bottom of the canister - or someone else left one in there - that's the usual cause of 'the oil didn't drain away' problems. By the time the canister lid is unscrewed enough to start lifting off, the oil should have drained away.

[RobZR]

Also the canister lid has tabs that lock into the top of the filter
if these are broken or cracked and don't lift the filter up as you unscrew it, it will not drain.

[Jamos]

The filter came up with it as I lifted, I think that's why oil spilled out, it just pulled it up with it as I lifted.