I have a chimney with the basement's woodburning insert's pipe running
up parallel to an abandoned vent for an indoor grill. The grill's vent
on the chimney is capped with about a 1" thick concrete block caulked
in place.

One wet winter when I got the woodburning insert hot enough, lots of
water vapor (and some accompanying smell) was driven into the house
through the old indoor grill location.

I don't know if water seeped through the concrete cap or if it seeped
in through the red brick. The overall poured chimney cap seems OK.
Should I try to apply a sealer of some sort to the small cap block?
Other ideas?