I stopped taking any budgie outside after my first parakeet, Inochii, died. Long and sad story...

All that you've mentioned, Kksuns, is very good. I, too, doubt that anyone has stolen them. In truth, they probably flew away. Keep calling to them. Maybe you could put on bird sounds. A cage is supposedly consider a 'safe haven', and they usually go back to it. Set the cage out with the door open and they might come back. All of Kksuns ideas are good.

Good luck!