online airline/car rentals-first try cheapest?

I recently had to book a flight and car rental to Washington DC.
I am amazed at the price differences between the initial online inquiries
and subsequent attempts. My desired car rental quote more than doubled
after a second inquiry. That is, I went to several car rental places (including Hertz)
and through Expedia. I booked a car for a good rate with Hertz through Expedia. I
visited each site only once.

I went back to Hertz and then Expedia to see if I could modify the times. Even using
the original times the cost at both sites went up significantly, through Expedia it doubled.
There were only a few minutes between the visits to the sites.

I believe I saw something similar with the airlines.

The price difference for both the car and the airline were literally hundreds of dollars.
I am amazed.

Have others seen similar behavior? Do sites raise their rates if they think you are indecisive or are shopping around?

It could be you changed something in your search but try a different browser or clearing out your cookies & search again & see what you get.

I think Spiderman might be right.

I read a piece online about a year ago, perhaps on Lifehacker or some similar site, that said that many online travel vendors and other similar businesses offer better prices to people who have not shopped with them before, in an effort to get you on board as a customer. If they can see, either from cookies or from you logging in to their site, that you’re an existing customer, they might be less likely to give you the best price.

I don’t know how true this assertion was, or how widespread this behavior might be, but it certainly conforms with what seems to be the predominant attitude in business at the moment: It’s more important to get a new customer than to treat the ones you already have with courtesy and fairness. (See also: the cellphone, cable TV, and internet service provider industries)

Your suspicion is correct at least with airline tickets but I would suspect car rentals work the same way. You should always price airline tickets using a different PC or your browser’s incognito mode. If the site detects you checking again it will be higher as a pressure to get you to buy. I have read this in more than one source and have seen it with my own eyes.