Any bird lovers here? I need help ASAP

I recently put up a post on FB advertising that I will care for pets. I had a nice couple pay me to care for their birds while they were off on vacation. I'm not too familiar with pet birds, but I gave it a shot. They were being extra loud this morning so I decided that they might want some fresh air. I let them all play outside when i let my dog out this morning. I cant find two of them now.

I don't think this is on me though. The owners said that the birds CAN NOT fly. But I can't find them anywhere.

They were not left outside by themselves long, maybe 15-20 mins.