On 01/16/2016 2:20 PM, John G wrote:
My family owns a single story house that no one has lived in for a few
months. ...

I stayed at the house during the holiday season and noticed that the first
time I flushed the toilet, the shower drain gurgled. I know that that can
happen if the trap is empty, so I ran water in the shower and then flushed
the toilet again. No more noise for the rest of the 4 day stay.
....

Well, my sister just called and told me that she was at the house, flushed
the toilet (the first time since Christmas) and heard the gurgling in the
shower. She ran water in the shower, flush again and heard no more noise.
(She was not aware that I had had the same experience about 3 weeks ago.)

As best she could, she checked the trap in the basement and didn't see
any signs of leakage.

Since the house was empty for a few months before the sewer work was done,
....

The traps could be drying out by evaporation. The place has been
empty in between gurgles, consequently no water was going down the
drain during those times.

+1 as first choice if there's no visible leak

Although only a month to six weeks seems pretty quick, the water surface
level only has to below the bottom level of the U by a fraction for
there to be an air break so it's not like the trap has to be completely dry.

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