I've been thinking about the Delta detail all day...
IMO, LE was a mere 1.5 hours into the investigation at the time of MP's interview. LE had a dead Doctor, a bloody hammer, and a husband out of town. I think at that point in the investigation LE was just collecting facts and establishing a timeline, in addition of course to processing the scene.
MP discovered TS's body just before 10 AM. MP was interviewed by LE from 11:35-11:52 AM.
LE may have determined at that point that TS had flown Delta (receipt, luggage tag, paper boarding pass/eboarding pass on phone, whatever) and were perhaps confirming this info with their witness.
It's certainly possible that MP learned that TS had flown Delta based on evidence at the crime scene (a boarding pass, luggage tag etc...) but he had already admitted to shaking her body (Bates 5106) during the 911 call and placed himself at the scene. So I don't think it's related to what MP saw/didn't see when he discovered TS.
I strongly suspect that at less than 2 hours into the investigation, LE wanted to confirm TS's itinerary so they're could retrace her steps. At that point in time LE may have suspected they had a staged crime scene but they wouldn't have known yet that MS is a



that hired a hit. For all LE knew, the killer hid in TS's van or followed her home. So retrieving surveillance video would have been a priority. Since parking at the airport TS used can be dependent upon the airline one flies, this detail would be significant.
They may have suspected initially that MP was somehow involved, he did after all discover her body. So they may have wanted to determine if he knew TS's travel schedule prior to MS calling him to check on his wife. But they didn't ask MP to divulge his whereabouts for the 12 hours prior to finding TS. In the 911 call he stated that he last saw TS 2-3 weeks prior.