Return immediately on equality and omit the final array inspection:
def searchEarly(u: Array[Int], z: Int): Boolean = {
  var a = 0
  var b = u.length
  while (a < b) {
    val middle = a + (b - a) / 2
    val value = u(middle)
    if (value == z) return true
    if (value > z) a = middle + 1
    else b = middle
  }
  false
}
An equality returns a genuine matching entry. After a nonequal comparison, descending order excludes the inspected middle entry and one whole side. In the half-open range, setting b = middle already excludes the middle index; subtracting one would incorrectly exclude another possible entry. The candidate interval decreases as before, so absence is reported exactly when no possible entry remains.

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