My dog keeps barking at a vent in my foyer and has been doing it for days. We took him into the basement and he just keeps sniffing and we searched the area up and down stairs and he won’t stop. (He hasn’t been eating or drinking as much over this and isn’t going out as much as he used too)
If he’s not eating and drinking so much, he has less need to eliminate, all of which adds up to not going out as much as he used to. Others suggested why he may not be eating and drinking so I won’t try to guess that. Showing him the basement won’t stop him from being exciting about the vent because he doesn’t know that the vent leads downstairs. For him to sniff a new area of the house is normal behaviour for a dog—sniffing is a dog’s way of exploring new territory in the same way that humans want to look and touch every aspect of an interesting new place or object so his sniffing the basement tells me nothing.
As others suggested, you yourself may want to investigate the vent itself to check for vermin. Anything that moves—from spiders and ants to squirrels and cars—can get dogs excited. Also, getting your attention and getting you excited can get him excited, thinking the whole thing is a wonderful game, great fun. To correct that idea tell him “No” when he barks at the vent and take him away from the area. Be sure to give him enough attention and exercise away from the vent, when he’s not barking at it, so that he does not think he has to bark at the vent to get your attention.
Once he realizes that barking at the vent is not a game he may forget about it. If there really is something (mice? ants? spiders?) in it, he won’t be able to leave it alone and you will want to eliminate the vermin.