Upvote:5
The sign of the cross is a sacramental (a sacred sign instituted by the Church, not one of the seven sacraments, but still a way to bring one's focus onto holy things, to become more prayerful in the moment). Specifically this sign is a reminder of our Baptismal promises (because we invoke the Trinity) and is a reminder of the cross. Stephen Ray writes:
The Catholic Church has always seen outward gestures as means of expressing and actuating internal spiritual realities. Sacramentals, such as the Sign of the Cross, are not superstitious practices but are sacred signs by which various things in life are rendered holy through the effectual and sacramental grace of God. By the Sign of the Cross we pledge allegiance to Christ and invite the Holy Spirit to apply the cross to our livesβto take up our cross and follow Christ.
We can only guess what the frustrated person you encountered was thinking, but one reasonable hypothesis is that she might have made the sign as a silent prayer for God's help to resist the temptation to get angry.