Thank you for posing an excellent question! As a Roman Catholic (Western Church) attending a Maronite Catholic liturgy (Eastern Church), I have also seen some parishioners make the sign of the cross and then bring their right hand up to their lips. Here is an historical perspective.
The Sign of the Cross is not traced on the body in the same way by all Eastern Christians. At the words, "... and of the Holy Spirit," the majority of Eastern Christians move the hand horizontally from the right shoulder to the left. This was the universal custom of the Catholic Church, East and West, into the Middle Ages. For example, Pope Innocent III in the 13th century directed that the sign of the Cross be traced in this way by all Catholics, with two fingers and the thumb of the right hand joined (see the New Catholic Encyclopedia [1967], p. 479). Sometime later in the West, the direction was reversed to movement of the hand from the left to the right.
The joining of two fingers and the thumb was a reaction to the Monophysite heresy, which denied the two natures of Christ, signified by the two fingers. With the addition of the use of the joining of the thumb and two fingers the Trinity is signified. This heresy began in the Syriac Antiochene area. Since Maronites defended the true teachings regarding the natures of Jesus, they restored the practice of making the Sign of the Cross with the two fingers and the thumb joined.
The Sign of the Cross is made at the beginning and end of all prayers. In the Maronite liturgy, it is also the response given when the priest blesses the congregation throughout the liturgy, using the Book of the Gospels, the Consecrated offerings and the handcross.
**Since Maronites defended the true teachings regarding the natures of Jesus, they restored the practice of making the Sign of the Cross with the two fingers and the thumb joined.**
I've never heard of this!