Pour water into the other side drain and see where it comes out. It probably ends above the plastic panel under the rocker panel area.

If it is like some Volvos that I have owned, it is not too hard to take this panel off.

The rivets might be plastic with a small pin in the middle. You push this pin in until the pin falls out and then the rivet comes out. If you catch the pins, you can insert them into the rivet a bit and they will expand the rivet again. Try one when it is out of the car. Buy a rivet from a dealer to see how it works.

I use a length of high temperature stranded wire - its insulation is stiff, but not anywhere nearly as stiff as a coat hanger.

I check all my drains by pouring water down them about once a month. (I park my cars outdoors under some trees.) If you have a sedan, there may be drains in the front corners of the trunk.