Best hiding place is in plain sight. I can't tell you how many times I can't find something when it's right in front of me! XD
Then run it through the garbage disposal for good measure.I'd microwave it.
Air handler is the unit inside your house, not a bad idea though.Install a fake electrical connection near air handler outside with a cover plate it would be a pipe into ground with a 90 turn that has a cover plate. Then pipe part way into house if you already have one for legit reason use that.
Just drill the top of the door so you wouldn't have to take the door off, a side note most interior doors are hollow inside now days.Inside the house: Remove a bedroom door (three pins). Drill a 3/4" hole in the bottom of the door. Stash the stick in there. Plug with a wood dowel. Put the door back on.
Outside the house - Dig a 2 foot deep hole with a spike point crowbar. Drop the USB stick in there. Cover with dirt. Such holes are usually very small in diameter, and do not create fresh dirt.
Hopefully any 'electronic' odor will be minimized in case they bring in dogs.