#define PIR 2
#define IR 3
#define BUZZER 8

unsigned long lastTime;
const unsigned long LIMIT = 30000; // 30 sec

void setup() {
  pinMode(PIR, INPUT);
  pinMode(IR, INPUT);
  pinMode(BUZZER, OUTPUT);
  lastTime = millis();
}

void loop() {
  if (digitalRead(PIR) || digitalRead(IR)) {
    lastTime = millis();          // activity detected
    digitalWrite(BUZZER, LOW);
  }

  if (millis() - lastTime > LIMIT) {
    digitalWrite(BUZZER, HIGH);   // inactive alert
  }
}