I think i read in a body language book that children cover there mouths when embaressed, lying or shy as its like covering there faces/hiding. Adults are not as obvious when shy/embaressed but are more subtle and maybe touch their faces not as obviously. putting there fingers in there mouths migth be similar to putting there hands over there mouths maybe? x
I wouldn't call it covering as such? I've noticed when shy that Mini Boo will put one or two fingers in her mouth, and since noticing her doing it, i've also clocked loads of other children do it when shy too.
Just guessing BOO, but as someone who sucked their thumb until I had my own kids (age 34!), I think it is a comfort thing in the same way - I was also very shy, not so bad nowadays - can't really be shy when you have kids.