Before us is Mishnah 6 in chapter 3 of Maseches Taanis, containing two incidents in which the elders decreed a fast on the basis of warning signs alone.
The first incident - blight in Ashkelon:
"Maaseh she'yardu zekeinim mi'Yerushalayim le'areihem ve'gazru taanis al she'nir'ah ki'melo pi tanur shidafon be'Ashkelon" - the elders left Yerushalayim for their own cities, and they decreed a fast because grain was found that had been struck by blight, that dryness which damages the grain and is liable to become a problem for everyone. Even though the quantity found in Ashkelon was small - only enough to fill the mouth of an oven - they decreed a fast on the basis of this sign alone.
The second incident - wolves in Transjordan:
"Ve'od gazru taanis al she'achlu ze'eivim shnei tinokos be'ever haYarden" - on the other side of the Jordan River, wolves ate two young children. This occurrence was considered unnatural, and therefore they decreed a fast over it.
The view of Rabbi Yosi:
"Rabbi Yosi omer: lo al she'achlu ela al she'nir'u" - according to Rabbi Yosi, no children were eaten at all. The very fact that the wolves were seen in the city, and that they appeared to display unusual boldness, is in itself sufficient reason to decree a fast.
In summary: in this Mishnah we learned of two incidents in which the elders decreed a fast on the basis of minimal signs alone: enough blighted grain to fill the mouth of an oven that was seen in Ashkelon, and the eating of two young children by wolves in Transjordan - and according to Rabbi Yosi, the mere presence of the wolves in the city was enough to warrant decreeing a fast.