Candidates are often referred to as either 'active' or 'passive'. But just how passive are 'passives'? Might it be more accurate to think of part of the passive group as 'receptive'? Receptive, as opposed to 'absolutely-not-looking-happy-in-my-job-thank-you', which consequently makes that group of people difficult and time-consuming to engage with.
There are many 'receptives' out there. You have possibly been one yourself at some point in your career. Sort of content in your role, not actively looking for a new job, but if something 'came up' you'd explore. The LinkedIn feature which allows people to switch on an 'open to job chats' signal in their profile is a gift to the passively receptive group - they are not in a hurry for a change so can wait for a Recruiter to find them.
Receptives don't actively look at job boards, but might not have switched off the email job alerts. Receptives may not have signed up with a recruitment consultant, but do keep their LinkedIn profile up-to-date. Just in case. Receptives are not actively searching various companies' career sites, but an interesting piece of content in their social feed or an event could catch their attention.
So, you need to consider what would pique your passive/receptive audience's curiosity and where you have opportunities to engage with them? This is a long-term strategy but the reward could be converting 'receptive' talent into an active candidate pipeline.