This is more difficult than is sounds. Herpesviruses are quiescent (latent) for much of their lives and exist only as DNA, with no virions being produced. There are different latency "programs"--during some of these, there may be a handful of virus genes produced, and some there are no genes produced. Circumstances may cause these virus genomes to reactivate--transcribing genes and making virions.

Maybe confusingly, herpesviruses may still contribute to the biology of the cell even in the latent state.

DNA/RNA sequencing is also more sensitive in some ways (which can be both good and bad). It is also a very broad tool. You can go back to large datasets and try to squeeze more information out--even things you werent looking for originally, like herpesviruses. NCBI also hosts a very large repository of raw sequence data (SRA--the sequence read archive). Much of this data has decent metadata, so you could conceivably even do a new study on somebody else's old data.

Realistically, probably because they're not virologists. Looking for nucleic acids is easy/commonplace. Virus culture, electron microscopy and immunoassays are more specialized techniques.

If they aren't virologists, maybe they should leave this kind of research to those that are? The headline says "Researchers Find Herpes Viruses" ...but they didn't (or cannot?) find them? Then, umm...why does it say that?