Check your lower radiator hose clamps..

07 Ridgeline.

Did the radiator replacement, my fittings were about ready to fail. Sent the whole deal to a friend who works for another manufacturer; they're interested if Honda isn't.

Had a slow coolant leak for some time (even before rad replacement) that I couldn't trace. Finally got motivated and checked everything. Dye revealed that there was a leak that showed up under pressure but only after the system cooled down a bit at the lower connection to the radiator; the lower clamp had corroded enough to lose it's spring force and would have eventually rusted through. It was easily removed by hand.

If you're in there, replace the clamp. Hose looked fine. I put a stainless worm clamp on until spring when I'll put new hoses and OEM clamps in. Leak fixed!

I thoroughly soaked the whole area down in rust check green to avoid future problems.