Serial1 prints to itself !?

Hardware
1

Im attempting to send a byte array to a grundfos serial sensor, but before I can do that I encountered a problem.
Whenever I use Serial1.write( any value ) it will be returned read on Serial1.read() instantly afterwards.

The error occurs even when there are just wires connected to TX/RX on the photon, and nothing at the other end of the wires. My code is the following:

int counter = 0;

void setup() {
  Serial.begin(9600);

  // Used to check if host has serial port (virtual COM port) open.
  while(!Serial.isConnected()) {
    Particle.process();
  }
  Serial1.begin(9600, SERIAL_8N1);
}
void loop() {
  counter = counter + 1;

  if (counter % 10 == 0) {

    unsigned char telegram[5] = {
      0x72, 0x05, 0xEE, 0x03, 0x68
    };

    Serial.println("Sending...");
    // Serial1.write(telegram, 5);
    Serial1.write(42);
  }
  delay(1000);
}

void serialEvent1() {
  while (Serial1.available() > 0) {
    int inByte = Serial1.read();
    Serial.printlnf("Read %X", inByte);
  }
}

Console output:
35

2

How long are your wires?
Your symptom description sounds very much like your RX wire is picking up the TX signal - open wires can quite well act as antennas.

3

They are only like 5 cm each. I would need to place them further apart?