"the brownies"

Turns out she's was the only one that passed the drug test.

To spoil the "joke" even if marijuana is legal in your state this is technically a boobytrap, which is illegal.
Do not do this.
You, not the thief, can/will get fired and arrested.
This applies to all "don't eat my food" things - drugs, laxatives (technically poisoning as well), super duper hot sauce, etc. As much as breakroom lunch thieves deserve a place in Hell, don't get yourself in trouble, bring it up repeatedly with HR until they put a camera in and catch/fire the thief.

wow so just literally get someone fired over a couple fuckin brownies? real mature karen