Is this a porch? They look like HVAC vents roughed out so the porch could be enclosed later.

And you're talking about doing a fire hazard there with a grill underneath a building structure. If you do that, you will need one of those giant all metal exterior grade hoods over it and ducted straight up and out to get rid of the smoke and to provide the required non combustible surface over the grill.

'Possum portals.

What's above them? Just a porch roof? Second story? Attic? If you poke a snake in, do you get any clues? Is it a tract home where you might be able to get some info from neighbors?

"When the house was constructed, there were two HVAC units."

Which means that you also had two comfort heat systems.

I would speculate that these are high and low combustion air for the respective systems.

If there are still two comfort heaters, in the same location, there are available from plumbing supply houses, other than the big box's, a non-toxic smoke bomb, about the size of a pop-it fireworks, which are used to find leaks in air circulation systems.

Detailing the method, if the heaters are no longer there, would be a waste of time.

Snoony,I think you may have nailed it! I sent the photo to the HVAC company who installed the last unit. The manager said that he also thinks it's the high low vent and no longer necessary. Therefore we can cover it up.

Green D- I understand what you're saying, however the entire area is open. We've lived here for decades and our bbq has always been in this location. Its all good. I will however make sure that we place the bbq island a good distance out from the wall.

Good.

Generally these screens were made of 1/4" pattern, galv. screen mesh. Clean them and if so, using a hammer and wood block cave the frame in slightly, apply stucco and paint.

Nice crossbuck windows, by the way.Typical of the period arch.