According to 1701 UNIFIL is there to assist the Lebanese Army in stopping arms smuggling, not to interdict arms shipments itself. Even when it comes across an arms depot its mandate is such that it must inform the Lebanese Army of that discovery and then ask if the Lebanese need UNIFIL's assistance to remove those weapons i.e. UNIFIL can not remove those weapons itself. Go read 1701, and you will see that this is true.
U.S. says working on initiative to de-escalate Syria fighting, with focus on Aleppo (Reuters)
from the article: UN chief warns Israel-Lebanon tension could lead to new conflict