I’m glad you found him.
Mystery Snails, Pomacea bridgesii, are notorious escape artists. They can fit through very small gaps in the lid/hood of your tank and are surprisingly good at hiding.
How big is your tank? What kind of pleco did you say you had? I’m only asking because if it is a common pleco, it will outgrow your tank faster than you would think.
I suggest looking for any gaps in the hood where he could get out next time. Does he has sufficient food? If not he was trying to... 1( Get more food. 2( Find a mate. 3( Wanting to get out because the water quality it bad. What are your water parameters?