The drain to my bathtub runs out the floor of my apartment and below me is the garage space with exposure to the current cold weather. Somewhere in that pipe was standing water that froze and became a plug, preventing the water from draining.
Wednesday, I got tired of waiting. I used a common household substance to resolve the issue.
Any guesses? Answer below.
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Salt. From making ice cream when I was a kid.
It should be insulated if it’s in an exposed area. P traps should & will have water in them to keep the sewer gas from leaking into habitable space.
That is also a cold garage. Lol. When it gets this cold out, you might consider heating the garage slightly. Unless of course its a parking garage.
I am reminded of my visit to Alaska. We inquired about whether the homes we were observing were habitated full-year or seasonally. The person we were speaking with stated they are inhabited year-round, with various “fail safes” among the neighbors to ensure survival. He followed this up with “it’s a good winter when you can keep the sewer running; everything after that gets easy”. 
Steve,
Thanks for the verification that you use the bathtub keeping those odors away!
Cheers!
I rent…
“Public” garage.
I would mention that spray in / on expanding foam insulation material is pretty impossible to trace back to whomever might have applied it… it just doesn’t hold fingerprints at all…