So my wife was home and I was at work at the time this event happened. She told me after I got home, there was a man in a green vest with a camera, taking pictures of our house. We're curious as to why this man was doing that, as well as what the green vest could mean.

Any info appreciated!

This is a global website. How would we know what a green vest means?

Maybe he is a private investigator. Maybe he is a realtor. Maybe he liked your house but had poor fashion sense.

Is there a loan on the home, sometimes, the lender has an inspector.

Who is your insurance company? They may do a home inspection?

Could the home be in foreclosure. It is common for the banks to do photos of homes in foreclosure.

Is there any code violations? Could it be an inspector?

Did anyone consider going out and asking him?

I saw someone doing this once, and I went out and asked. I was told 'To list a house just like yours.' HUH? I said 'Get lost.'
Of course if he wasn't actually on your property, you can't do anything about it.
I didn't like it for several reasons, one being who wants dishonest listings out there? My house wasn't just like another.

The entire world is on google earth. You can opt out by using their link to do so. But they don't send people out on foot. All by car and airplane and satellite.

Could be a tax assessor. Daylight green is commonly worn. Could be a surveyor-did your neighbor just sell their house?

that is a very likely event, when houses are valued, they often use other homes that are similar.

On the down side it could be an enemy or stalker or P.I. or just some random dude amateur photographer. Or a skew of other reasons could have inspired him. If he comes back, ask him what's up. You can call the local assessors office to see if someone was sent out. You may also inquire with your landlord/mortgage holder to see if they sponsored the activity. You call also call around to a bunch of local realtors to see if they sent someone out for a comparison assessment.

If you're really concerned you can file a police report, which will accomplish nothing, but you will have a record of the event and if repeated or related to any crime, will be documented for further prosecution.