So I thought I'm sure about this and my instincts say that: "If anyone has seen them .." would be right but then again when I said it like: "If anyone have seen them .." I started thinking which one would be the right one. Can you please guide me through which is the right one and why? P.S. English is not my first language.
It's "if anyone has", because "anyone" functions as third person singular. It probably just seems right to use "have" because you would for any other number or person.
If you break the word down as "any one", you can tell which is correct:
if any one has seen them