Tips for subtle hints in characters?

Hello! I'm a first time dm and I'm planning on running a campaign with a secret cult running in the background. The plan is for them to see multiple members over the course of the campaign and slowly connect them to being a group, but not quite figure out what they're doing (unless they catch the clumsy one towards the end of the campaign.) I just don't know how to give subtle clues to show they're part of the same group, or that there's a cult operating at all.

One idea I have is their cryptic promotional flyers, where it will have their symbol and some religious message about who they worship (Tiamat). What else can I do?