Vias under Power Transistors
Normally, a power component like this would have a big pad and some heat dissipation space under the heat slug. It might even have thermal vias going to another heat pad on the back side of the board. If it's being run at anything close to it's full current capability, that would be a requirement. This one is probably a bit underutilized though, rendering the big pad un necessary. It's still a good idea for mechanical strength, but in the prototype world, we don't always follow the rules.
The problem here though, is that the open vias under the part can short to the thermal pad. That's bad mojo. Some board fab houses can put a solid coat of mask over the vias and they'll be okay. Not all board houses will do that though, and if the board has a silver finish (this one is HASL), you wouldn't want the vias sealed because silver vias can outgas and corrode if completely sealed.
We just put something non-conductive between the part and the board and it's all fine, but as I've said so many times before, that's not best-practice, even for a proto, and it's doubly not good in a production environment.
Connect the 16 via dots and what do you get?