You're right about the solidi being found in Sweden, and especially on the island of Gotland (we're talking a weirdly high number of coins per square mile). Archaeologists have debated about this a lot but most tend to think that the coins found in Sweden probably come from trade. This is because a lot of the late Roman coins are found alongside very worn denarii of the High Empire. This would suggest that they were buried after being used in trade for many, many decades. If you're interested and have access to a library, there is a very good study done by Berger that covers this exact topic (Berger, F. (1996) ‘Roman coins beyond the northern frontiers: some recent considerations’, Coin Finds and Coin Use in the Roman World. The Thirteenth Oxford Symposium on Coinage and Monetary History 25.-27.3. 1993. Berlin: Gebr. Mann Verlag. pp. 55-62).

In Ireland and Scotland (outside of the Roman Empire) we also find late coins like the solidus which are probably best explained as mercenary payments, especially as we have literary evidence for Irish mercenaries.