Since you rarely use the bathroon the water in the toilet trap probably
evaporated over time alowing sewer gas in, it will happen faster with
sinks , tubs or any drain with a trap. If the smell went away it was a
dry trap, If the smell doesn`t go away it could be a bad wax ring. The
hardest to find and fix is an enclosed cracked vent stack. But since
yours went away it was sewer gas comming in from a dry trap, just flush
once in a while.