Saw this the other day under my truck bed
between the bed and the frame
is it something that wasn’t removed before assembly?

 

Forgot to mention it’s got some sort of adhesive on it but obviously it has failed.

 

Probably sound dampening stuff.

 

Looks like sound dampening.
It is only that one small piece under the entire truck?
Unless there is something specific under the mat making noise (ground side, not body side) it seems odd there is only one small piece. Why there are the preventing noise to seem into the empty bed. ha

Makes the bed sound less hollow when walking ont it?

 

Just that one 1x3
I think it’s some sort of heat shield because it’s above a component of the exhaust

 

That seems logical.
The material is suitable for heat but apparently the adhesive has a lower temperature rating. LOL

 

I have one too but it is still attached. Apparently it is a heat shield as someone else noted and your adhesive didn’t stick too well. Mine isn’t on there too great either.