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I would guess that a spammer posted one instance of the links. And once the thread was viewed by members the warnings were triggered. I would then say a couple of your members re-posted the links to inform you, and that caused more instances to show up. I seen this happen before. The best thing your members can do in a case like this to to just inform you of it.

If a spammer did not post links, then I would say your server was compromised and your host removed/cleaned any files that were infected. I had this happen to me before and my host denied their server was compromised. But when a few other sites on the same server complained too, then you know they were compromised. In my case I changed hosts, not because they were "hacked", but because they denied being so.