To smell something fishy means to realize that something is not quite right and that there may be a specific reason for that.
This idiom is different from To smell a rat.
1. When Monica did not look directly at her mother while talking about the picnic, she smelt something fishy.
2. The boss realized something was fishy when not a single employee turned up on time for the meeting.
3. The coach smelt something fishy when the entire hockey team reported sick for training.