We had our bathroom remodeled and now smell sewer gas in the bathroom. It started the day they finished the job, and we never smelled it once before that. It comes and goes, and has nothing to do with how often the sinks, shower or toilet are used. We never smell it in the winter, and only during warm summer days - never at night.
The job entailed moving a single sink to the other side of the bathroom and making it a double sink. Toilet and shower were replaced but stayed exactly where they were. The only time I remember the plumber saying anything about the venting was when he asked us if we were OK with him running another vent pipe up through the roof, rather than tying the new ones into the old vent pipe (we said it was OK)
Any ideas what it could be? I did take pictures of the new piping before they put the floor in and sheetrocked.

Sniff around the floor joint with the toilet base. The wax ring could be missing or not thick enough.

I know your plumber asked if you want a new vent pipe for the new sink, but did you actually see him put in the pipe and made sure it went all the way out threw the roof. If he just vented it into the attic that could be why you smell it on warm days with all the heat that is created in the attic on warm days it usually make sewage smell worst. So if you can see the roof where the vent location is you should be able to see two vent pipes coming out within a few feet of each other.