I've noticed that my boss is acting too friendly with me, giving gifts, and insisting that we don't make our conversations public. Could this be grooming if we're both adults?

Yes, grooming doesn't only happen to children but also in relationships where one person has power over another, for example, a boss over a subordinate. If he insists on keeping the relationship secret, gives expensive gifts for no reason, asks you not to tell anyone, and wants to spend time alone, these are warning signs. Grooming is a slow process of blurring boundaries and creating a false sense of care to make it easier for you to comply. So trust your intuition: if you feel uncomfortable, talk to someone you trust. If you need more clarity, seek professional psychological help.