You cannot put a tracking pixel on a private Slack recommendation. That does not mean dark-funnel activity is unmeasurable, just that you have to measure it in proxies instead of clicks.
Signal one: branded search volume
When people search your company name directly instead of a generic category term, something outside your tracked channels put your name in their head first. A rising trend in branded search, independent of any paid campaign, is one of the cleanest dark-funnel proxies available, and it's free to watch in any search console.
Signal two: review site traffic and profile views
G2 and Capterra both show profile view counts. A steady climb in views with no matching increase in your own ad spend toward those platforms means buyers are arriving there through word of mouth or a peer recommendation, not your marketing.
Signal three: the first honest answer on a sales call
Ask every new customer, in the first call, 'how did you first hear about us,' not the last click before they converted. Log the raw answers verbatim. Patterns across a dozen answers, a specific community, a specific person's name mentioned twice, are a map of your real top-of-funnel that no dashboard produces.
Signal four: deals that arrive pre-sold
Track how many discovery calls start with the prospect already knowing your pricing, your positioning, or a specific feature you never advertised directly. A rising share of pre-informed calls against flat website traffic is a strong sign that dark-funnel activity, not your outbound, is doing the real qualifying work before the call even starts.
Build a monthly proxy-tracking habit
None of these four signals require new software. Pull branded search volume from your existing search console, review-site views from your existing profile dashboard, and the other two from a single recurring question added to your sales process. Reviewed together every month, they won't tell you exactly which podcast or Slack thread moved a specific deal, but they will tell you, with real confidence, that the channels you can't see are working.