I think when you see the cop car go into the area in front of the terminal, you can spot Cash's car.
It doesn't cease to amaze me,in this and other episodes, the ready availability of on-street parking in busy city areas.
The Goof section on this web site even references visibility of Columbo's car before the murder.
I think the critical point here is that Columbo concluded he was coming back soon; the "planted seed" did take root.
Long term parking wouldn't make sense. Why still pay the rent? Besides, I think Columbo indicated he first noted the rental tag on the keys when Cash went through the metal detector. Cash himself had made the statement he was going on an extended tour. Columbo thought he wasn't going, then he was, then he wasn't. The logic is simply he was going for a lengthy period and the rental keys being maintained meant he was coming back soon. The fish was hooked. He had to move that parachute before the cub scouts etc. combed the area for the missing thermos.
Well I think my point here is that if he was going to be gone for several months, he surely would have turned in the rental car. If Columbo saw it anywhere OTHER than the rental lot where he turned it in, that should have been the clue - not the keytag that he didn't notice until he was at the security check point insdie the airport.
Also, did you notice how easy it was for Columbo to go through security even though he "beeped?" Try that today at any airport and the TSA will have your butt in a sling. Even if you do flash a quick badge.
Your point is logical, but all I'm saying is that it's not evident that Columbo was merely a car length or so behind him. He could have been several minutes at that point. It's not evident to me that Columbo saw the parked car one way or another. They wrote it a certain way simply that the vision of the key triggered a question or contradiction in Columbo's mind. I for one love episodes where extraneous incidents occur, and you're not quite sure what is important to pick up on. I'm sure we could all write different scenarios which we think are better than what was done.