Can anyone please tell me how to find Harriet whilst patrolling the halls??
Hard to tell from the question. Assuming from description that you already caught Charlotte smoking twice. If you have not overheard Harriet in Cafeteria, do that (means you have to open it if you haven't yet). To make her next patrol event appear, you need to have topless punishment passed already.
If you mean where do you find her after all the patrols, she's in the gym Monday and Friday afternoons. You will need Rachel with you as Pet and full body search rule passed for that.