[krash]

If you search, upi can find a lengthy post I made last yeat.. water accumulates in the exact same place in the back corners you circled... and when they fill up it would spill over, probably sloshing when I drive, into the wheel well and sit trapped under the tire.
No rust in the spot you show..

Never did find the source, I did drill a couple drain holes in the corners, coared with rust prevention primer, and painted to at least allow the water to drain... Ended up buying a car cover that is water-proof.

 

[krash]

I went out and checked mine, not an Si, but the '08 EX Cpe, has a wing like lip on the back edge of the trunk... I thought long agon that might have been where the leak might be.. on the EX there is a thin rubber gasket between lip/spoiler and the trunk, mine is wone and half gone... that lip on mne is held on by 4 bolt/nuts and a couple plastic clips.

Looks a little different than your photo but if its the mount, looks like yours may be missing one of the plastic push/pop in clips or the sted for the nut is broken off.

 

[krash]

I got tired of chasing it, and I don't give up easy same as when i had a short draining the battery,.. ended up buying a cover and nor more leaks, just can't take the cover off and drive in rain storms... I'm retired now so I can pick my days when & where I drive.