It shows up because dealers don't clean their scales. Those benzos and other pills begin as powder before being pressed. Also, pretty much all black market oxys or Percocets are pressed fent these days from what I've heard. Unfortunately, it takes very little fentanyl to cause an overdose, especially in those who don't have any previous history with opioids (i.e. kids taking mdma at a music festival not expecting to stop breathing) . Cannabis laced with fentanyl is nonsense.

Edit: I'm going to copy this and post it as a root level comment so others can see.