Sounds like you're good, just close it back up and sit tight. Every brew is different.
I did the same thing this morning with a Wit I brewed this weekend. No airlock activity after 72 hours, so I popped the top expecting to have to pitch some US-05 I keep as a backup. But instead I saw some krausen and got a harsh whiff of CO2 when I stuck my head in there.
What made you think it wasn't fermenting? No bubbles in the airlock? Could just be the bucket wasn't completely sealed and the C02 was escaping elsewhere.
Bottled/Drinking: Apfelwein, Leaping Goats Coffee Brown Ale (v1 & v2), Anniversary Barleywine, Ginger Brew, Spring Cleaning IPA, High Ground Gose
Primary/Secondary: Sour Blackberry Cider, Vanilla Rhubarb Mead, Brett Saison