I don't really want to block most ads, only the really obnoxious ones, and it doesn't make sense to add almost every single website I visit to an exceptions list, so I have uBlock Origin running in blacklist mode: you know, where I see the matrix of things it blocks, and the top left cell, labeled "All", is in dark green, and all the cells below that are in light green, unless I choose to block a specific element.

This seems to work fine, and on most sites that I visit, all the cells of the matrix are green, and the information panel on the right tells me "Blocked on this page 0 (0%)". Yet a number of these sites still give me a pop-up telling me that they have detected that I am using an ad blocker. If I click the power icon to add those sites to uO's exception list and reload the page, they are satisfied.

But how did they detect it in the first place? As I said, I'm talking about sites from which uO is telling me that it hasn't been blocking any ads (since it's in blacklist mode). Is uO blocking things that it doesn't tell me about (but then why doesn't it tell me about them)? Or is there some other sneaky way that sites have to detect a running ad-block extension even if it's not blocking anything they're trying to serve (but then why doesn't it detect them when them when the site is added to the whitelist)?

I'd be grateful for any enlightenment.