Is it permissible to be cautious and wary of a Muslim if he exhibits behavior that arouses suspicion, without spying on him, or does that fall under the forbidden [evil] suspicion?

It is not permissible to harbor ill-will towards Muslims without proof. However, it is permissible to be wary of certain individuals if strong indicators suggest that harm may come to you from them, without resorting to spying or falling into excessive suspicion. It is advised to rely on Allah and frequently remember Him, especially through the remembrances for protection.