I recently noticed a sewage smell coming from my shower drain in the guest bathroom. My situation is a bit different than others I've seen on this site because it's been weeks since anyone has used the shower and it probably hasn't been used more than a dozen times since we moved here 9 months ago.

When I noticed the smell, I was about to take a shower but hadn't turned on the water. I put my nose to the drain and it was definitely coming from the drain.

The house is a ranch-style home with no basement (we live in Florida).

Is it possible for sewage to back up into the shower drain?

Yes it is possible to have sewage smell because you are have not used the shower quite often, the trap in unused fixture has evaporated all the water out of it leaving it open to sewer gas.
Pour ½ gallon bleach in the drain, Allow to sit overnight and then use a bucket of boiling water in the morning.
Or use Ballengerb1 (Bob’s) recipe Mix 1/2 cup of table salt with 1/2 cup baking soda dump into the drain followed by a cup of vinegar, Allow to sit overnight and then use a bucket of boiling water like in the morning.
When you done with the bleach or Bob’s recipe keep the shower in use or keep the trap not to dry, good luck.
John