Does God use signs? Yes.
14 And God said, Let there be lights in the firmament of the heaven to divide the day from the night; and let them be for signs, and for seasons, and for days, and years: (Genesis 1:14)
Why does God use signs? For a witness.
4 God also bearing them witness, both with signs and wonders, and with divers miracles, and gifts of the Holy Ghost, according to his own will? (Hebrews 2:4)
Is there an example of when God used signs in the past? Yes.
1 Now when Jesus was born in Bethlehem of Judæa in the days of Herod the king, behold, there came wise men from the east to Jerusalem,
2 Saying, Where is he that is born King of the Jews? for we have seen his star in the east, and are come to worship him.
3 When Herod the king had heard these things, he was troubled, and all Jerusalem with him.
4 And when he had gathered all the chief priests and scribes of the people together, he demanded of them where Christ should be born.
5 And they said unto him, In Bethlehem of Judæa: for thus it is written by the prophet,
6 And thou Bethlehem, in the land of Juda, art not the least among the princes of Juda: for out of thee shall come a Governor, that shall rule my people Israel.
7 Then Herod, when he had privily called the wise men, inquired of them diligently what time the star appeared.
8 And he [Herod] sent them to Bethlehem, and said, Go and search diligently for the young child; and when ye have found him, bring me word again, that I may come and worship him also.
9 When they had heard the king, they departed; and, lo, the star, which they saw in the east, went before them, till it came and stood over where the young child was.
10 When they saw the star, they rejoiced with exceeding great joy. (Matthew 2:1–11)
In this example, the sign was supplemented by a prophecy and it was Herod who provided it.
Are there any prophecies of God using a sign in our future? Yes.
30 And then shall appear the sign of the Son of man in heaven: and then shall all the tribes of the earth mourn, and they shall see the Son of man coming in the clouds of heaven with power and great glory. (Matthew 24:30)
25 And there shall be signs in the sun, and in the moon, and in the stars; (Luke 21:25)