I know that if I spend enough time without doing anything suspicious, things calm down, but what happens if I'm too suspicious? Will I return to prison?
For each of the Primary Menaces (Nightmares, Scandal, Wounds, and Suspicion) there is a consequence upon reaching level 8 with any of them.
For Suspicion, once you reach Level 8, you are hauled back to New Newgate Prison, where you'll be stuck until you reduce your Suspicion to 0.
Before that reach that point, however, you will notice some other events related to high Suspicion.
For instance, once your Suspicion reaches 4, you'll see the "Find a way to throw the Constables off the scent" Storylet appear at your lodgings, and give you an opportunity to ask your friends for help.