What if a bathroom doesn't have vent pipes on the roof above it?

You can vent them in a different way: http://en.wikipedia.org/wiki/Drain-waste-vent_system

However, the traps should still be keeping that smell out. Maybe there’s gunk in the traps?

Edited: this link: http://en.wikipedia.org/wiki/Drain-waste-vent_system#Air_admittance_valve

Each drain doesn’t have to have a unique and separate vent that pierces the roof. That bath vent is pobably tied in with the original vent pipe. It can also use a studor vent in some locations. It’s a small mushroom shaped cap, possibly in the cabinet under the sink. As previously said, the trap under the fixtures should be holding water to stop sewer has from coming up. Is the bathroom used often? The trap water will evaporate if not used for a while.

Thanks, both. The bathroom was in fact not in use for six months, but after running water into the drains and also pouring down the good old home remedy of baking soda and vinegar, a gassy smell remains. I’m calling the septic guys in the morning.