They - whoever "they" are - have a variety of stunts they pull.
It used to be with vBulletin, we could check our profiles to see who the last 10 visitors were to our profiles. That's how a couple of them got caught - I noticed a new account with a suspicious post count, as in 0, visiting my profile. A brand-new stranger would have no reason to do that.
Ditto with the "like" system. I don't mind when legitimate members drop a "like" on an old post of mine, but if a brand new, zero-post count account does that, my spambot detector (similar to a BS detector) goes off and that account would appear to be one needing investigation.