Could small recurrence of inguinal hernia be detected by ultrasound. (note surgery was done with mesh in open style)?

No: Neither ultrasound nor any other imaging test is ideal to detect any inguinal hernia, whether small or large. These are found with a physical exam by your surgeon. The only time in which an imaging test can detect an inguinal hernia is if something gets stuck in it (i.e., incarcerated). Go see your surgeon.

Hernia: A small recurrence could be detected by ultrasound, ct scan and/or physical exam. The lack of visualized hernia, though, doesn't necessarily mean that there is no recurrent hernia. These can be difficult to detect, even by experienced surgeon examiners. Discuss with a surgeon.

Possible: Most of the time recurrences can be felt, if there is pain likely to be a nerve entrapment in the scar, will eases with time, if no symptoms best management is observation if it is getting larger.