I found this neat little helmet, the 'Underhelm' it adds +1 to search, but unlike all other helms, nothing to concentration. Was this intentional or a mistake?

Deliberate. Sort of.

Hundreds and hundreds of bits of ambient loot that randomly pop up places, I don't put a whole lot of thought into each one.

Hello,

I too also found an Underhelm, this one says it has 50 charges but not what the charges are for nor can you activate the helm to use said charges. Also, it can't be sold to merchants.

I was wondering if this treasure item is bugged?

The charges are a bug (sort of, misleading at least, although it doesn't make a difference). Pretty much no magic items are intended to be sold to merchants, as the price merchants offer for the kind of loot sprinkled around would be vastly too high for the gold/magic level.

So, no, no real bugs there.