Provided they are installed correctly and actually vent outside, overhead vents are a million times more effective than popup vents or the old JennAir ones where there is a vent built into a cooktop. I don't think that venting is required by code unless you're talking a super high powered range/cooktop, but you'll regret it if you don't have it. There are proper vents that can be installed over an island, but they are generally more expensive and require more labor to install and duct to the outside.

OP, is this a stove in a house you might be buying? Not all stove fans are immediately visible, so you might just have missed it. Our stove has no overhead venting system; it vents from the bottom.

I dunno but you can get vents that are built into an island.

Are venting systems above or near stove required to meet code in MD or did I just see this on a show or something? And if a house has a stove on an island and there is no vent, does that spell trouble (smells, cleanliness?)...I've never not had a fan so I am wondering if this is an issue.