You are right. I checked my email it I did receive an email on the same day saying "Your Hilton Honors account has been updated" - I had booked a reward night stay two days before and I had assumed it was just a delayed notification that points had been deducted.
Logging into the system to check my points activity there was a an event for a transaction for 0 points as you said. I tried the online chat as suggested by
@Jonova and they were able to provide me with the certificate number.
I still had to call up to make the reservation though as they could not do it online.
This was a bit annoying because I'd been looking out for a different email similar to what I had received before telling me I got my reward night and that also tells you your certificate number.
Anyway, on a side note, I would usually check online to see if there was availability to use my reward night. The rule I thought was in place was if there was availability for Standard Reward bookings you could use your certificate, but today when I checked there was zero availability on ANY date for the next 8 months (I didn't check any further). But when I called to make the booking there was availability for the date I wanted.
The bottom line is even though the system may suggest there is no availability it may be worthwhile ringing to check anyway just in case there is some glitch like it seems to be at the moment where it's saying there is no availability for any night at all.