Is your toilet and sink, etc in there vented out through the roof? If so, is the vent disconnected somewhere. This sounds very much like the smell, from an unvented plumbing, of which toilet would be the worst. If you find that it is vented, and isnt disconnected somewhere, then climb up on your roof, and check out that vent pipe. Birds have been known to make a nest over the vent opening. You can also run a snake down that vent from the exterior, just to make sure. Sewer gas is not healthy to breathe, so its something you need to do asap.

I live in a townhome and we recently had our roof redone, so possibly something was done that is blocking the vent? I'm honestly not sure of the bathroom is vented through the roof but if it is, wouldn't this affect the other bathrooms as well?

Not all bathrooms share the same roof vent.

I called two drain companies to check a similar problem. Both refused to climb to the second storey, even though the vent was accessible without leaving the ladder. You may encounter similar scaredy-cat plumbers as well.

Like worthy said. Each bathroom, and kitchen and laundry room, should have their own vents. Venting is a requirement for all systems, so that sewer gas doesnt kill you. Venting into a crawl space, or basement, or attic is a huge no no, because sewer gas is still getting into your dwelling. It must go to the exterior of your house, and be up high.

It may be your vent, or could be your sink drain.

I found out my problem/cure from googling problem. We had recently replaced the sink stopper and faucet in bathroom. Shortly afterwards noticed sewer smell occasionally, few times a day. Yes, sewer smell is a skunky smell most times. My googling efforts said to check the overflow of the bathroom sink to see if clogged.

I checked by using a funnel and pouring water into it to see if water drains. Sure enough it was clogged. When installing drain stopper we got excess plumbers putty into the hole that drains into the pipe. We are amateurs at home improvement and something usually goes wrong/not smoothly. After some fiddling with something to clean out putty and water drained properly no longer had sewer gas. It was explained on the Google info the clogged overflow sometimes causes some weird suction when toilet is flushed causing sewer gas to come up...

Good luck.

Wanted to provide an update to everyone. I had a plumber come out yesterday. Of course I was unable to recreate the smell when they were here, however they hung around for a while as we flushed the toilet and still no smell. They didn't see anything wrong with the toilet or sink plumbing or any signs of leakage or water damage anywhere. They were pretty sure it wasn't a venting issue since I live in a townhome and I would have noticed the signs in other bathrooms.

One thing I forgot to mention in the first post was that the bathroom in question has a shower that never gets used. The plumber almost immediately asked me if the shower ever gets used, and when I said no, he identified the smell could be coming from the shower p-trap water having been evaporated. Long story short we ran a bunch of water through the shower and I can say that I have not smelled the problem since. So I'm thinking it was the shower p-trap.

Happy to hear you had a plumber, and hopefully, all it was, was an unused shower. Maybe try to use that shower every now and then, or at least pour some water into it, every other day or so.

Yep definitely going to run water through it once a month now.

Funny, I was going to suggest checking the shower before I read all the way through your story. Every so often, I have to pour some water down the drain in my basement shower to prevent it from drying up. When it gets dry, the sewer gas can escape from there somewhere. This is a very common problem but an easy solution.

You need to run water in it more often than once a month. At least once a week, would be so much better.