You should look for him in every space and corner you have there. I once had one, it escaped, and I found him three days later stuck to a shelf's leg, crouching tightly, and all covered in dust. Then, I rinsed it with lukewarm water, dropped it in a tank, and it started swiming swiftly.
I know some frogs do that, so it won't likely be seen crawling around. They also sometimes go to wet/humid places like the bathroom.
I have been lucky with escapees, my tarantula climbs up the glass when she is light enough, I am surprised no one has ever stepped on her, or any other venturous explorer.