I'm not intimate with the S90s, certainly not the 2017-on second generation FWD version. Your RWD 1997-1998 S90 is a follow on to the 960, a similar chassis to the 740/940s, but the 4-cyl engine compartment is laid out quite differently than yours. Until one of our S90/V90 or later 960 owners here catches this thread and is able to confirm it, here's what looks like a proper diagram of the early S90 evap system layout. It agrees with you seeing the evap line heading under the firewall toward the rear. The charcol cannister is shown underneath in front of the gas tank, although the shape of the cannister doesn't agree with the Volvo schematic I posted above. In any case, shouldn't be hard to find it once you (safely) get your head underneath.

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Dave -still with 940's, prev 740/240/140/120 You'd think I'd have learned by now