Test with Photon and WiFi commands.

wifiManagerTest.ino

/*
I'm testing what happens if a WiFi network is not found while I've put
in the correct creditials, but the network is offline (use case: router temporary unplugged).

Why does WiFi.hasCredentials pause for about 21 seconds? What is going on
in the background?

This only happens when WiFi.connect() is called before
(which doesn't influence the WiFi.connecting() flag directly it seems).

Serial monitor output:
start
WiFi.connecting() : 0
WiFi.ready() : 0
WiFi.hasCredentials() :
<here it will "pause" for about 21 seconds>
1
duration : 21634
WiFi.connecting() : 1
WiFi.ready() : 0
WiFi.hasCredentials() : 
*/

#include "application.h"

SYSTEM_THREAD(ENABLED);
//SYSTEM_MODE(SEMI_AUTOMATIC);
SYSTEM_MODE(MANUAL);

void wifiManager();

long duration = 0;
long startTime = 0;

void setup() {

  delay(5000);
  Serial.begin(57600);
  Serial.println("start");
}

void loop() {
  wifiManager();
}

/*

*/

void wifiManager() {

  WiFi.connect();

  Serial.print("WiFi.connecting() : ");
  Serial.println(WiFi.connecting());

  Serial.print("WiFi.ready() : ");
  Serial.println(WiFi.ready());

  startTime = millis();

  Serial.print("WiFi.hasCredentials() : ");
  Serial.println(WiFi.hasCredentials());

  duration = millis() - startTime;

  Serial.print("duration : ");
  Serial.println(duration);

  delay(100);
}