The whole point was scans were down UNTIL they slammed saucer into planet. They couldn't detect chapel due to the beam the saucer was used to destroy

Let’s wait until part two.

I think we may already have enough to figure out what happened but the technological explanation is yet to come. Much of the plot mechanics related to the Gorn so far rely on issues around what can be detected or transmitted and differences in solutions.

The writers’ challenge for the saucer subplot was that they wanted Spock to be surprised by both the adult Gorn in the environment suit and by Christine Chapel.

Their arrivals behind Spock on the exterior of the saucer were both unexpected, and were key elements of the suspense. His surprise and ours was necessary.

We would have expected however Spock to have done some kind of local tricorder scan of the wreckage when he arrived. It’s possible that a tricorder scan was done, was negative, but we didn’t hear any report because there were no vocal coms back to the Enterprise. Uhura gave a play by play based on telemetry, we didn’t hear Spock report directly.

In that case, we’re owed an explanation about why the new tricorder technology failed. As long as we get it in the second part, I’d be fine.

Given the established interference field technology of the Gorn, I would be perfectly comfortable if the follow up episode acknowledged that the Gorn environmental suits put out some kind of localized disruptive stealth.

The new Starfleet tricorder technology is designed for unsuited Gorn. It’s designed to solve the problem of Gorn biology but not Gorn technology.

Gorn technology is different, they are driven by different species biological imperatives (as in the coronal flares), and that’s an extra hurdle for Starfleet.

We have already seen however that Scott designed a system to both spoof human life signs to Starfleet tricorders and Gorn as well as hide human life signs for hundreds of people. To do this, he used some of the specialized technology from the scientific research array that was studying the nearby sun.

Spock would naturally follow up on his surprise encounter on the saucer. Scott would be the natural collaborator to figure out how it was that the Gorn came up behind him undetected by his tricorder.

So then, what about Chapel in the saucer? If she was the sole human life form, and he completed the scan, why didn’t his technology detect her?

A couple of possibilities exist.

-- Chapel’s suit has some local stealth technology. She got into her suit as soon as she saw Spock pass by. Given it was in her quarters it’s a personal suit not a generic one, and she’s established as being a war veteran who had to fight despite being medical corps, and/or

-- the distortion field or stealth technology put out by the Gorn’s environmental suit was large enough to hide her as well.