Oh, ok makes sense, btw can poets and entertainers be spies?
I think all the visitor types can be spies, yeah. New version added a bunch more types to ensure spies get to move around a lot, but, yeah, think spies get to be all of them new and old.
Monster Hunter spies are easy to spot. They're the ones who don't petition you the moment they arrive. Apply magma.
Note that petitioners can spend a very long time praying in dedicated temples, if you have those, before hauling their sorry asses to actually petition (I've had performers and scholars spending more than half a year praying and whatnot before actually showing up to petition). This means that praying Monster Hunters may just be religious nuts as well as suicidal regular nuts...
Absolutely sure about that?
Monster Hunters arrive with the intention of holding a meeting unlike any other visitor types. They're the one kind which don't seem to want to hang out first (but will do a lot later).
Or maybe it's just that I hit caverns before building a temple.
I'm sure about what I stated. The "might" for Monster Hunters is intended to indicate uncertainty. Any it's incorrect that Monster Hunters are the only ones to arrive with the intention to hold a meeting. That goes for every visitor who intends to petition: as you're able to see their intentions, they state the (possibly fake(?)) reason for their visit, such as Long Term Work, but that can be in addition to visit dedicated temples (not sure about relaxing). Regular visitors' stated intent can be to relax and to visit a temple, perform and to visit a temple, or research and visit a temple (and I think the new temple targeting types can come to visit the temple and to relax).
If visitors are spies, it doesn't make sense for them to claim they're going to petition and then don't when they can just claim to visit to relax/pray (this doesn't apply to Monster Hunters, who are supposed to visit only if they intend to slay monsters, so any Monster Hunter who does not claim to be there to petition is probably a spy).
So far I've failed to find any spies, but I know I've been visited by ones, as gobbo sieges have skipped over traps. I'm still looking for (DFHack) means of identifying them. Once I've found one to examine I can decide what to do from there.