After a few days after taking delivery of my Prius I noticed that in the glove compartment there was a sachet containing 2 rubber plugs and on it was mentioned -
Important: To Pre-delivey Inspector. Insert these rubber plugs into the holes of the center floor side member extensions. The part number was 90950-01550
There is also a diagram shown on the rear where these plugs should be fitted on the underside. I tried to check under the car to see if there were any holes that these plugs should be fitted on but couldn't exactly find where these need to go.

Does anyone know what is the purpose of these plugs and if any of you have come across these and tried to fit them on your own. Should I just leave it as it is. Also do I need to check with the dealer why these were not fitted during the PDI. The dealer is a bit far away from where I live so would like to avoid taking it back to him to have these fitted.

I've read about something similar (maybe in the RAV section) and I think it was to do with the holding down points used for shipping. And the plugs you have just blank off these points. Don't quote me, it is just a vague memory.

Anyway, when you have time just have a good look under the car to see if you fit them in somewhere.

look just in front of the rear wheels underneath.
go along the chassis rail and you should see the holes

I did a thorough check on the underside including the chassis rails but could not find any empty holes where these plugs could fit in. Maybe whoever has carried out the PDI have probably used plugs from their own stock and not bothered to use the ones that came along with the car. Did speak dealer about them too, but the person I spoke to wasn't sure about the spare set in the dashboard. They mentioned they the PDI is done as per the checklist provided by Toyota.

In case any of you haven't had these holes covered with the plugs, I can provide the spare set that I came with mine. I guess it is important as it reduces incabin noise as well as presents water and other debris from entering inside the cabin.