In Plain Sight by Brian Patten.

I rode up to the border on my donkey.
The guard pulled me aside.
“What’s in those sacks the donkey’s carrying?
What have you got to hide?”
I didn’t like his tone of voice
Or what his questioning implied.
I said there was absolutely nothing
That I wished to hide.
Still, he searched the creature’s panniers,
And it really was a farce
When he snapped on a pair of surgical gloves
And looked up the donkey’s arse.
Finally, he gave up searching.
He decided what I’d said was true.
He waved me on way. I smiled
And smuggled the donkey through.