So I was looking for a hotel award in Paris at the end of August, and Ren Vendome had a standard reward available on the day I was interested in. Logged in, chose my room type, looked at the pre-confirmation screen... but before I entered my credit card details and confirmed it, I thought "Nah, how about I change the dates and try the next day instead?"

Alas, there was no availability (even StayAnytime) the next day, so I cycled back to the previous day... and whomp, the standard reward was gone, only StayAnytimes left. I went back to the main page, tried again, same result. Log out, log back in, try again, same result. I tried going back to the original reservation session from the browser history, but nope, that was gone too. In fact, "Show Alternate Availability" showed me that standard rewards were only available on one single day during 4 weeks of Aug-Sept (which was, of course, the same day when I have a confirmed evening engagement in another country 2000 miles away).

"Hmm", I thought, logged out and did something else for a few hours. Then I tried my luck once more... and lo and behold, the standard reward was back. Not wanting to push my luck, I booked it immediately and got it too. Out of curiosity, I checked again today just as I was typing this, and the week with the day I booked is now 'awarded out' but there's a Friday night the previous week that most definitely wasn't there under 24 hours earlier.

It thus looks like just looking at reservations is enough to trigger locks on inventory; the locks aren't tied to your session; and that the locks aren't specific to the day but seem to cover longer periods. Moral of the story: if an award you were looking at seems to disappear into thin air and nearby dates all seem full, it may be worth waiting an hour and two and checking again before tearing your hair out in despair...