`05-`10 (3rd gen.) are known for this. Just fixed my `06 Odyssey EXL a month ago. Check hatch weatherstrip, but mine was the roof seam at the rear, both sides.
Mine was leaking at the cracked roof seam seal at the top of the rear hatch jam. Mine is definitely fixed. I dried out the rear cargo area, and since we have had crazy rainfall and the cargo area has remained dry.
Most of the cracking was visible with the rear hatch open at the very top besides the hinges on both sides. The crack is super small , but cracked/separated in a way that aided the water to run in.
I did remove the roof rack and the black trim pieces to check and seal under there also. But almost all of the cracking was the last ~4-8 inches of the seam. And most/probably all can be sealed without removing the roof rack.
If you need to. Roof rack comes off easily. End caps slide off after pressing release through hole. Then some torx screws and nuts if I remember right. Took longer to clean out the channel than take the roof rack off.
I used GM seam sealer (not easy to work with) after digging out the cracked dried up sealer with a screwdriver and pick. Have paint thinner available for clean up of excess seam sealer.
Make certain area is 100% clean and dry so seam sealer adheres.https://www.amazon.com/gp/product/B007Q0ZQGK/ref=ppx_yo_dt_b_asin_title_o03_s00?ie=UTF8&psc=1
Maybe look for a smaller more manageable tube of seam sealer. Only a small tube is needed.
Following day, touched up with Honda OEM touchup paint. Tricky area to get into. Paint won't match because the jams are just base coated with no clear, but at least looks better. Mine is Slate Green Metallic.