If you didn't find anything, keep looking.
Same thing happened recently to an unimpeachable site that I know slightly. At first they simply assumed minor hacking to scare them into buying some unneeded security software. The culprit ended up being a Russian site with contact info in Lithuania; I remember looking them up and thinking that RIPE's verification criteria were due for an overhaul.
It was educational for a reason you may not even have thought of: I was surprised at how many browsers independently use g###'s security verification. The "may harm your computer" text doesn't only show up in SERPs* but as advance warning in the browser itself.
* Gosh. I had no idea this acronym was invented by anyone in particular ;)