Residents of southern Israel heard air raid sirens on Monday evening, although there were no immediate reports of rockets landing in the region.
The IDF said it was investigating the cause of alarm in Sderot and the surrounding Shaar Hanegev area.
There was an unconfirmed media report of a bullet apparently fired from Gaza that pierced a car and also damaged a home near the border.
In the past, the air defense sensors have been triggered by machine gun fire, which the system took for an incoming rocket. Such may have been the case on Monday as well.
Earlier in the day, Israeli soldiers and Palestinian gunmen clashed on the Gaza border, where the rioting of past weeks has ebbed, but such incidents contribute to a still jittery situation.