being at his level and "seeing" where he could be able to reach from there can help

a pic of the room might come in handy to give you some hints.. so we know what furniture, hidden enclosures, in between stuff places we can suggest..

if he hid it in 5 secs.. it probably was somewhere in the low part.. under something, near something (like leg of bed,dresser etc).. if he had to do it fast.. low hiding would be easier.. because as I see "high" hiding could be easy for you to find

have you checked bathroom?.. tub?.. back of toilet?