First of all,check your filter.Bristlenoses,and other Pleco's are known for showing up in filter's and such.It's gotten to the point where I have to take apart my filter weekly to get at least 10 or so out.The first time I realized this,I found 37 baby Bristlenose in the filter,and all were alive and well.
Secondly,do you have any openings on your tank lid,and have you checked around your floor in the tank area?I have also found and realized that when I keep my water level real high,that this has happened to me on occasion.
Third,I once bought 2 Baby Rubberlip Pleco's that were about an inch and a half-maybe two at the most.I decided to keep them with what I considered my "small" goldies.As soon as I added the Rubberlips to the tank,the nearst Goldie took a bite out of one thinking it was food. Even then I thought it would be okay.I had hidey holes and plants for them,and I thought the goldies would be okay.Well they weren't.One was gone by morning,and the first one soon died after the goldie tried to eat him.To goldies,they really can't tell what is food for them and what is not.They just see it and try to eat it.