Strange smell from shower and tub drains in upstairs master bathroom. No smell anywhere else in house. House is about 2 years old and we are its 1st owners. Could it be from the vent on roof for bathroom, and if so, how do I unclog it?

Unscrew your drain cover in your shower. Take a flash light and look inside. Do you see white "goo" accumulated inside / around the drain ?

If those go to the same vent, and those are the only drains linked to that vent it's a logical guess. It's not uncommon for birds or other critters to get in there, not be able to get out and die. If you can get to the vent opening and have a tool to get down in there and try to "grab" the offending object you could give it a try. There's a tool for that type of thing. It comes in various lengths, made of flexible, coiled wire. It has a little "plunger" on the handle end that you puch and it pushes a little claw-like hand out of the other end. Release the plunger, it clamps on, and you retract the line and see what you've got. A plumber could also try to snake it. If you don't attempt it yourself I would think a plumber would be the repairman of choice. It if turns out not to be the vent and he has to go to work on the drains themselves that's who you'd have to call anyway.