How often is water run in the sink in the guest bath? Your trap could be evaporating, allowing sewer gas into the room.

Is there are vanity in the bathroom? Something underneath or behind it?

I 2nd the idea that you must run water in the sink, bathtub, and flush the toilet on a regular basis or you can develop odors. Also what is the foundation under the bathroom? check that area and also outside of that room (inside and outside of the house).

Thanks everyone for your responses. Our bathroom has the toilet and shower area have a separate door from the double sinks/mirror part of the bathroom.

We've definitely found that the smell is coming from the shower/toilet room.

To answer a couple questions:

-We have concrete slab floors.
- We have completely removed the toilet and put it in the garage and the smell still persists.
- The shower is one big fiberglass unit with no signs of leaking.

Thanks again for the suggestions. Does any of that information help the diagnosis?

Have you checked above the ceiling? How about heater and exhaust vents? Look and light fixtures, too. I once found a dead bat in mine :o

Sounds like a trap issue allowing gases to come back into the room. In the elbow pipe, water sits and blocks that but there are a few things involved- pitch and distance to the vent stack. If anything is blocked or not pitched, vented right, that can easily cause the issue you are describing. Key: Find a "good" plumber. This is what they do.

Maybe mildew/damp smell from the concrete floors? If you had an especially wet winter maybe the concrete is wicking moisture? Here is an article that might have an explanation and solution.

Also, I once had a mouse build a nest in the insulation of my fiberglass shower, but it entered from the crawlspace under the house. That might be impossible with a concrete floor, unless there could be access another way. It was a foul smell though, not weird...mouse urine from the nest - ugh!

http://www.instructables.com/community/How-can-I-get-rid-of-the-smell-of-damp-concrete-in/

Have you snaked your shower drain? Any time we have had a nasty/weird smell that persists in the bathroom, we've tracked it down to a giant glob of nastiness (hair, toothpaste, shampoo, soap) in either the sink or shower drain. Removing the drain cap and snaking out the nastiness is a job for someone with a really strong stomach, but it has always solved our stench issues. Good luck!

We had an awful smell in our son's room over the garage when we first moved into our house. We could never find what it was and I always wondered if there was a dead animal in the walls. After a few years - yes, that long, it went away. We now believe that during construction the insulation got wet from rain and that was what cause the odor.